fix(cli): remove AGP 9 property shims and jcenter() in cap migrate

[andredestro]

Description

Extends cap migrate with two Android cleanup steps that Gradle 9 / AGP 9 make necessary:

1. gradle.properties: removes the 10 compatibility flags that Android Studio's AGP Upgrade Assistant writes to preserve the AGP 8 defaults. android.useAndroidX=true is kept.
2. jcenter(): removes it from android/build.gradle.

Both steps run before the gradle wrapper upgrade. The wrapper task runs on Gradle 9.5.1 the second time around, so a leftover jcenter() would fail it at the configuration phase and the wrapper upgrade would be lost.

mavenCentral() is not added back, and android/app/build.gradle is not scanned, per review feedback: the 3 -> 4 migrator always added mavenCentral() regardless of the opt-in jcenter() removal, it has been in the template since Capacitor 4, and jcenter() has only ever been in the root build.gradle of the templates.

Adds cli/test/migrate-android-gradle.spec.ts: 15 tests covering the transformations and a block that runs them against a project on disk.

Change Type

• Fix
• Feature
• Refactor
• Breaking Change
• Documentation
• Other (CI, chores, etc.)

Rationale / Problems Fixed

Users have been running the AGP 9 migration in Android Studio while still on Capacitor 8. The Assistant injects flags that a Capacitor app does not need: most are deprecated and will be removed in AGP 10, and a couple break a Capacitor 9 build (android.builtInKotlin=false disables the Kotlin support AGP 9 now bundles, android.sdk.defaultTargetSdkToCompileSdkIfUnset=false stops AGP from inferring targetSdkVersion).

RepositoryHandler.jcenter() was removed in Gradle 9.0. Any build.gradle that still calls it fails immediately at the configuration phase with Could not find method jcenter().

Platforms Affected

• Android
• iOS
• Web

[jcesarmobile]

I don't think migrate command should add mavenCentral(), it was already added in the 3 to 4 migrator, the deletion of jcenter() was optional, but the addition of mavenCentral() was always done, so we could assume all apps have it.

jcenter() has never been in android/app/build.gradle in our templates, only in android/build.gradle, so I don't think we should worry about that neither.