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/sonar.yml

@@ -27,11 +27,11 @@ jobs:
           # Ensure we are building the branch and not the branch after being merged on develop
           # https://github.com/actions/checkout/issues/881
           ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.ref }}
-      - 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: