Raise Java version

Author: JulianKniephoff

.github/workflows/m2.yml

@@ -16,7 +16,7 @@ jobs:
         uses: actions/setup-java@v4
         with:
           distribution: temurin
-          java-version: 17
+          java-version: 21
           cache: maven
 
       - name: See if we have the Opencast artifacts

opencast-backend/pom.xml

@@ -111,7 +111,7 @@
         <configuration>
           <failOnWarning>true</failOnWarning>
           <compilerArgument>-Xlint:all</compilerArgument>
-          <release>17</release>
+          <release>21</release>
         </configuration>
       </plugin>
       <plugin>