feat: upgrade build environment to JDK 21 and centralize CI configuration

- Create a composite GitHub Action `gradle-setup` to encapsulate code checkout, wrapper validation, JDK 21 setup, and Gradle caching logic.
- Update all GitHub workflows (`publish-core`, `codeql`, `scheduled-updates`, `release`, etc.) to utilize the new centralized `gradle-setup` action.
- Upgrade the project's primary JDK requirement from 17 to 21 across `jitpack.yml`, workflow files, and build-logic conventions.
- Refactor `KotlinAndroid.kt` and `build.gradle.kts` to target JVM 21 for the application while maintaining JVM 17 compatibility for published library modules (`api`, `model`, `proto`).
- Introduce a new `build-desktop` job in `reusable-check.yml` to verify desktop artifact assembly during CI.
- Implement dynamic `cache_read_only` detection in workflows to optimize Gradle cache usage across different branch types and merge groups.
- Update project documentation (`GEMINI.md`, `AGENTS.md`, `CONTRIBUTING.md`) to reflect the JDK 21 requirement and provide guidance on Robolectric configuration for the new version.

.github/workflows/docs.yml

@@ -47,15 +47,8 @@ jobs:
           submodules: 'recursive'
           ref: ${{ inputs.ref || '' }}
 
-      - name: Set up JDK 17
-        uses: actions/setup-java@v5
-        with:
-          java-version: '17'
-          distribution: 'temurin'
-          token: ${{ github.token }}
-
-      - name: Setup Gradle
-        uses: gradle/actions/setup-gradle@v6
+      - name: Gradle Setup
+        uses: ./.github/actions/gradle-setup
 
       - name: Build Dokka HTML documentation
         run: ./gradlew dokkaGeneratePublicationHtml