Remove JDK 21 as it's no longer available via Oracle's setup-java step

Author: marcphilipp

.github/workflows/cross-version.yml

@@ -18,7 +18,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        jdk: [21, 22]
+        jdk: [ 22 ]
     name: "OpenJDK ${{ matrix.jdk }}"
     runs-on: ubuntu-latest
     steps: