chore(ok_http): migrate to Built-in Kotlin and resolve AGP 9 warnings

[EchoEllet]

• Migrates to the built-in Kotlin Gradle plugin (per this guide).
• Migrates from Gradle Groovy to Kotlin DSL (KTS).
• Updates Gradle wrapper of the example app.
• Updates Kotlin and Android Gradle Plugin versions.
• Updates analysis_options.yaml of the example app to exclude non-Dart files (automated by Flutter CLI)
• Updates Java source and target compatibility from 11 to 17
• Builds the plugin Gradle project with warnings enabled in the example app to be more consistent with Flutter plugins (e.g., image_picker_android).
  • This is not strictly necessary and can be reverted.
• Migrates from legacy Gradle plugin apply.
  • https://docs.flutter.dev/release/breaking-changes/flutter-gradle-plugin-apply

The versions of Java, Gradle wrapper, Kotlin, and AGP have been updated to match the versions defined in the Flutter template 3.47, to be more consistent with packages by the Flutter team and newly created Flutter apps.

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[brianquinlan]

Can you resolve the merge conflicts?

[EchoEllet]

@brianquinlan
I have resolved the conflicts.

The updated example seems to work

[brianquinlan]

Thank you!