Migrate example Gradle config to Flutter plugin DSL

[matux]

Description of the change

Fix Flutter example Android build by migrating to Flutter’s Gradle Plugin DSL (remove imperative app_plugin_loader.gradle/flutter.gradle applies, move plugin versions to settings.gradle).

This unblocks flutter build apk on recent Flutter versions (3.16+).

Note

Fix fully generated by gpt-5.2-codex (high) with supervision

[Copilot]

Pull request overview

This PR modernizes the Flutter example Android build configuration by migrating from the legacy imperative Gradle setup to Flutter's declarative Plugin DSL, enabling compatibility with Flutter 3.16+. The migration removes manual script applications and consolidates plugin declarations into the settings file.

Key changes:

• Replaced imperative app_plugin_loader.gradle and flutter.gradle applies with the new Flutter Plugin DSL
• Centralized plugin version declarations in settings.gradle using pluginManagement
• Updated CI workflow to test against Flutter 3.38.5

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
rollbar_flutter/example/android/settings.gradle	Migrated to pluginManagement block with includeBuild and declarative plugin versions
rollbar_flutter/example/android/build.gradle	Removed obsolete buildscript block (plugin versions now in settings.gradle)
rollbar_flutter/example/android/app/build.gradle	Replaced imperative apply statements with declarative plugins block
AGENTS.md	Added new repository guidelines document for contributors
.github/workflows/ci.yml	Updated Flutter version to 3.38.5 for CI testing

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