Fix Package.swift deprecation warnings (package(name:url:from:), revision)

[BerkeleyDude]

Updates Package.swift to use non-deprecated Swift Package Manager APIs:

• Replace .package(name:url:from:) with .package(url:from:) (identity derived from URL).
• Replace .package(name:url:.revision(...)) with .package(url:..., revision: "...").
• Update target package: references to use URL-derived identities: AppAuth-iOS, app-check, gtm-session-fetcher (others unchanged).

Resolves the deprecation warnings when resolving/building with recent Swift toolchains (Swift 6 / Xcode 16+).

Made with Cursor

[google-cla]

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[ImanMahmoudinasab]

Thanks @BerkeleyDude ❤️ , this will resolve #576

[w-goog]

@mdmathias, this LGTM. It implies "dropping" support for Swift 5.1, but I think some of the SwiftUI bits in the GSI button are already 5.2+.

[w-goog]

@BerkeleyDude - You'll need to take care of the CLA before we can submit this PR :)

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[w-goog]

We're going to submit #579 instead, because I think getting Cursor to sign the CLA will be....tough 😅