Run CI on JDK 21 for Robolectric SDK 36 tests

[Copilot]

The build GitHub Actions job was failing in :app:testDebugUnitTest because the workflow ran Robolectric tests targeting SDK 36 on JDK 17. Robolectric 4.16 supports SDK 36, but requires JDK 21 for that test path.

• Root cause

  • CI provisioned temurin Java 17
  • Robolectric tests explicitly target SDK 36:
    • ExampleRobolectricTest
    • GreetingScreenshotTest
  • This mismatch caused the failing build job

• Change

  • Updated .github/workflows/ci.yml to use JDK 21 in the build job
  • No app, Gradle, or test logic changes

• Workflow update

  - name: Set up JDK 21
    uses: actions/setup-java@v4
    with:
      distribution: 'temurin'
      java-version: '21'

• Why this is scoped correctly

  • The failure was caused by CI runtime configuration, not production code
  • Aligning the workflow JDK with the SDK 36 Robolectric requirement resolves the failing path without broadening repository changes