Upgrade the used JDK in the project to v21 (#3582)

* Upgrade the used JDK in the project to v21

* Use it for CI too

* Centralise java language version

* Fix deprecations, tests and lint issues

* Fix coverage taking into account `@Preview` annotated code.

---------

Co-authored-by: Benoit Marty <benoit@matrix.org>

.github/workflows/sync-localazy.yml

@@ -12,11 +12,11 @@ jobs:
     if: ${{ github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == 'element-hq/element-x-android' }}
     steps:
       - uses: actions/checkout@v4
-      - name: Use JDK 17
+      - name: Use JDK 21
         uses: actions/setup-java@v4
         with:
           distribution: 'temurin' # See 'Supported distributions' for available options
-          java-version: '17'
+          java-version: '21'
       - name: Configure gradle
         uses: gradle/actions/setup-gradle@v4
         with: