Update Java version to 21

[ghentschke]

to fix the build errors

[jonahgraham]

If/when the build passes this is good for me to go.

[ghentschke]

Hm, it does not build. Does anyone has an idea to fix this:
Error: Failed to execute goal org.apache.maven.plugins:maven-antrun-plugin:3.1.0:run (generate-configuration) on project org.eclipse.cdt.debug.application: An Ant BuildException has occured: The following error occurred while executing this line: Error: /home/runner/work/cdt/cdt/debug/org.eclipse.cdt.debug.application/configGenerator.xml:4: java.lang.UnsupportedOperationException: The Security Manager is deprecated and will be removed in a future release Error: around Ant part ...<ant antfile="configGenerator.xml" target="generate.configuration" />... @ 6:74 in /home/runner/work/cdt/cdt/debug/org.eclipse.cdt.debug.application/target/antrun/build-main.xml Error: -> [Help 1]

[ghentschke]

I think we have to get rid of the security manager since its an error in Java 21.

[ruspl-afed]

in this case we need to rework from ant call to something better

[ruspl-afed]

in this case we need to rework from ant call to something better

Unfortunately it is possible to do only with 17-> 21 upgrade and some other changes, see #761

[jonahgraham]

With #761 these improvements are all covered and the build works on Java 21. Thanks @ghentschke for your efforts here.

There is a part of this change that can/should be applied if done fully, that is removing use of security manager.