[AGP 9] Update Flutter Template Versions

[jesswrd]

Bumping AGP to the newest stable version that is compatible with some corresponding stable Gradle and KGP version. Will bump the warn version (to AGP 9+) and the error version to (right before AGP 9).

Update to:
AGP 9.1.0 (can't update to newer stable AGP version—not even the patch version—due to lack of KGP support here)
Gradle 9.3.1
KGP 2.4.0

Fixes #189380

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Code Review

This pull request updates the default template versions for Gradle, the Android Gradle Plugin (AGP), and the Kotlin Gradle Plugin (KGP) across several configuration and test files. However, the feedback highlights critical issues: the specified KGP version (2.4.0) does not exist, and the AGP version (9.1.0) is unstable, both of which will break project builds. Additionally, the updates are missing corresponding required changes in other files, such as DependencyVersionChecker.kt and build.gradle.kts, to keep the configuration in sync.

[gmackall]

Bumping AGP to the newest stable version that is compatible with some corresponding stable Gradle and KGP version

Do there exist AGP stable versions which are newer, but which rely on unstable Gradle/KGP versions?

[gmackall]

is there a reason why we don't use 9.1.1 (everywhere, not just here)?

[gmackall]

Per offline discussion, I think both questions are answered by
https://kotlinlang.org/docs/multiplatform/multiplatform-compatibility-guide.html#version-compatibility

which at time of writing only indicates that KGP 2.4.0 is compatibile with AGP 9.1.0 (but not 9.1.1).

LGTM!