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.

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

.github/workflows/tests.yml

@@ -46,11 +46,11 @@ jobs:
       - name: Clone submodules
         if: ${{ github.event_name != 'pull_request' || github.event.pull_request.head.repo.full_name == 'element-hq/element-x-android' }}
         run: git submodule update --init --recursive
-      - 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: