Unpin package:intl (#169286)

Fixes #158050

Created by running `flutter update-packages --cherry-pick=intl:^0.20.2
--exclude-tools` - we probably don't want to unpin `flutter_tools`
dependencies to avoid breaking it.

Add comment to `package:intl`s pubspec in
dart-lang/i18n#980.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: mosuem

packages/flutter_localizations/pubspec.yaml

@@ -13,11 +13,11 @@ dependencies:
 
   flutter:
     sdk: flutter
-  intl: 0.20.2
+  intl: ^0.20.2
   path: 1.9.1
 
 dev_dependencies:
   flutter_test:
     sdk: flutter
 
-# PUBSPEC CHECKSUM: c2j4ng
+# PUBSPEC CHECKSUM: pmbisj

pubspec.yaml

@@ -120,7 +120,7 @@ dependencies:
   http: 1.4.0
   http_multi_server: 3.2.2
   http_parser: 4.1.2
-  intl: 0.20.2
+  intl: ^0.20.2
   io: 1.0.5
   isolate: 2.1.1
   js: 0.7.2
@@ -213,4 +213,4 @@ dependencies:
   pedantic: 1.11.1
   quiver: 3.2.2
   yaml_edit: 2.2.2
-# PUBSPEC CHECKSUM: suvu4j
+# PUBSPEC CHECKSUM: r0f5ng