[various] Update READMEs to reflect current OS support

[stuartmorgan-g]

• Updates all READMEs to list the OS versions currently supported by the latest platform implementations.
  • This is not necessarily the same as the OS version that the current version of the app-facing platform supports; for plugins where the app-facing package still supports Flutter 3.32, clients using Flutter 3.32 will get older versions of the platform implementations that are still compatible with iOS 12/Android API 21-23. However, the Flutter team no longer actively supports those versions, which is what we are choosing to reflect in READMEs. (In the future, we may want to move this information to platform implementation package READMEs, although that will make it harder to skim.)
• Updates all Android plugins that still listed a minSdkVersion of 21 to 24.
  • This is a no-op for clients; all of these implementation packages already require Flutter 3.35, which can only build for 24+.
  • This part is the Android version of [various] Update min iOS/macOS versions #10231, but no SDK changes were needed here. All the changes where we could make code changes were already done in [various] Scrub pre-Android-API <24 code #9851.

Fixes flutter/flutter#178757

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[stuartmorgan-g]

Version override: the gradle changes are a no-op, so don't need release. We could version and release the README changes (and normally would), but I don't think there's much value in releasing just for that since nothing is actually changing for clients, it's just making things more consistent.

[stuartmorgan-g]

This was a mistake; the previous release already made the requirement 3.35. (Probably when I updated the whole repo the example app was updated, and so the package was caught in the CHANGELOG mass-update dragnet.)

[stuartmorgan-g]

Same; it was already 3.35.

[gemini-code-assist]

Code Review

This pull request introduces a series of updates across various packages to align documentation and build configurations with the currently supported OS versions. The changes primarily involve updating README.md files to reflect new minimum supported versions for Android, iOS, and macOS, and increasing the Android minSdkVersion from 21 to 24 in several build.gradle files. Corresponding CHANGELOG.md files have also been updated. The changes are consistent and well-executed, improving the accuracy of the support information for developers.

[ditman]

(Some changelog tests complaining)

[jmagman]

That was fast, thank you @stuartmorgan-g!

[jmagman]

nit: there's extra leading whitespace for this bullet for all CHANGELOGs

Suggested change

	* Applications built with older versions of Flutter will continue to
	* Applications built with older versions of Flutter will continue to

[stuartmorgan-g]

It's a nested list item :)

[stuartmorgan-g]

CHANGELOG override: The packages that are changed without changelog entries are the ones where the only change is minSdkVersion 21 to minSdkVersion 24, which is a no-op in practice for clients in the packages (as explained in the PR description).