[release/8.0.1xx] Detect correctly multitargeted apps for trim/AOT/si…

…ngle-file warnings (#35851)

Backport of #35767.

## Customer Impact
In .NET 8 we made a breaking change that introduces a warning when setting `IsTrimmable` or related properties in a .NET Standard library. The warning was introduced because we changed the behavior (to not reference the trim analyzers, and to not mark the assembly with `[assembly: AssemblyMetadat("IsTrimmable", "True")]`.

The intended way to fix this in a library was by multi-targeting the library to include a TFM supported by trimming, and to set `IsTrimmable` only on the supported TFMs. We got feedback (see #35528) that the warning was a painful break for multitargeted libraries, and that the warning shouldn't be produced in the first place for libraries following our guidance.

This change addresses it by silencing the warning for correctly multitargeted projects. The warning is silenced only when the multitargeting set includes a low enough supported TFM to ensure that the `IsTrimmable` assets will be consumed (instead of non-trimmable .NET Standard assets) in any supported app that references the library.

## Testing

Existing unit tests passed without changes. Added tests to validate the behavior for various multitargeted projects. Added tests to ensure that the changes don't break the behavior of projects that use custom `TargetFramework` values as aliases for `TargetFrameworkIdentifier` and `TargetFrameworkVersion`.

## Risk

Low to medium. This change reduces overall impact of the breaking change by limiting the scope of the warning, but adds complexity to the behavior. The complexity is unlikely to be surfaced to the developer, but is observable (it would be confusing to any developer who tried to understand the specific circumstances under which the warning was produced).

## Original description


The warnings introduced by
 #34077 and described in
 https://learn.microsoft.com/en-us/dotnet/core/compatibility/sdk/8.0/trimming-unsupported-targetframework
 are unnecessary noise for projects that multitarget to include a
 TargetFramework that is compatible with
 trimming/AOT/single-file, and is a low enough version to ensure
 that it will be picked over any other TFM's assets when the
 library is consumed in a trimmed/AOT'd/single-file app.

This fixes the issue by detecting correctly multi-targeted libraries and suppressing the warnings in these cases.

Note that prior to the .NET 8 SDK, netstandard libraries would still get the `IsTrimmable` attribute embedded in the assembly when setting `IsTrimmable`, and could use trim analysis despite incomplete warnings due to unannotated ref assemblies. With the .NET 8 SDK this is no longer the case. Even for correctly multi-targeted libraries, the netstandard output assembly will no longer contain the `IsTrimmable` attribute. We debated whether it was worth warning in this case, but decided that the negative impact of the warning on library developers following the "golden path" was too large to justify keeping this as a warning.

This solution addresses a correctness problem that we wanted to avoid (the correctness problem: allowing an inadvertently non-"trimmable" netstandard asset to be consumed in a trimmed app that uses a supported TFM).

It's still possible to get into that situation in an app that explicitly references the netstandard assembly, or that targets an EOL TFM (causing it to consume the netstandard asset from a library that multitargets `netstandard2.0;net6.0` for example).

Fixes #35528

src/Tasks/Common/Resources/Strings.resx

@@ -931,17 +931,17 @@ You may need to build the project on another operating system or architecture, o
   </data>
   <data name="IsAotCompatibleUnsupported" xml:space="preserve">
     <value>NETSDK1210: IsAotCompatible and EnableAotAnalyzer are not supported for the target framework. Consider multi-targeting to a supported framework to enable ahead-of-time compilation analysis, and set IsAotCompatible only for the supported frameworks. For example:
-&lt;IsAotCompatible Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net7.0'))"&gt;true&lt;/IsAotCompatible&gt;</value>
+&lt;IsAotCompatible Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', '{0}'))"&gt;true&lt;/IsAotCompatible&gt;</value>
     <comment>{StrBegin="NETSDK1210: "}</comment>
   </data>
   <data name="EnableSingleFileAnalyzerUnsupported" xml:space="preserve">
     <value>NETSDK1211: EnableSingleFileAnalyzer is not supported for the target framework. Consider multi-targeting to a supported framework to enable single-file analysis, and set EnableSingleFileAnalyzer only for the supported frameworks. For example:
-&lt;EnableSingleFileAnalyzer Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))"&gt;true&lt;/EnableSingleFileAnalyzer&gt;</value>
+&lt;EnableSingleFileAnalyzer Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', '{0}'))"&gt;true&lt;/EnableSingleFileAnalyzer&gt;</value>
     <comment>{StrBegin="NETSDK1211: "}</comment>
   </data>
   <data name="IsTrimmableUnsupported" xml:space="preserve">
     <value>NETSDK1212: IsTrimmable and EnableTrimAnalyzer are not supported for the target framework. Consider multi-targeting to a supported framework to enable trimming, and set IsTrimmable only for the supported frameworks. For example:
-&lt;IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))"&gt;true&lt;/IsTrimmable&gt;</value>
+&lt;IsTrimmable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', '{0}'))"&gt;true&lt;/IsTrimmable&gt;</value>
     <comment>{StrBegin="NETSDK1212: "}</comment>
   </data>
   <data name="Net8NotCompatibleWithDev177" xml:space="preserve">