These are the minimum required changes to update our templates to AGP 9.
Narrowed the scope from
[here](flutter#185730). This must be
CP'ed to beta and stable.

Bumped templates to AGP 9 to ensure all newly created Flutter apps are
on AGP 9 (and do not get the KGP warning).

AGP: 9.0.0
Gradle: 9.1.0
KGP: 2.3.20

Migrated app, module, and plugin templates from KGP to Built-in Kotlin
[Built-in
Kotlin](https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin).
Even though all necessary changes have been made to migrate to Built-in
Kotlin, Flutter does not support actually using Built-in Kotlin because
the default is still the legacy (by design).

Fixed some breaking changes when upgrading templates:
- android projects that contain similar package names fail to build
starting AGP 9
[here](https://developer.android.com/build/releases/agp-9-0-0-release-notes#:~:text=Enforces%20that%20each%20library%20has%20a%20distinct%20package%20name).
- compileSdkVersion was deprecated in AGP < 9 and completely removed in
AGP 9
[here](https://developer.android.com/reference/tools/gradle-api/8.11/com/android/build/api/dsl/CommonExtension#compileSdkVersion(kotlin.Int)).
- Java 17 is the new minimum for Gradle 9+
[here](https://docs.gradle.org/9.0.0/release-notes.html#jvm-17). This is
a no-op (except for unit tests) on our end because Java 17 has been the
minimum for AGP since AGP 8. Due to this, the warning message cites
Gradle version as the reason Java must be bumped instead of the AGP
version.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [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.
- [ ] All existing and new tests are passing.

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

If this change needs to override an active code freeze, provide a
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**Note**: The Flutter team is currently trialing the use of [Gemini Code
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