Switch to Java 21

[thll]

Graylog currently requires Java 17.

We should switch to the most recent LTS release, which currently is Jave 21. See https://www.oracle.com/java/technologies/java-se-support-roadmap.html.

Some preliminary PRs were created a while ago targeting the Graylog 6.0 release but were abandoned early once it was decided to switch after the 6.0 release:

• https://github.com/Graylog2/graylog2-server/pull/18197
• https://github.com/Graylog2/graylog-plugin-enterprise/pull/6577

The new PRs for Graylog 7.0:

• https://github.com/Graylog2/graylog2-server/pull/22908
• https://github.com/Graylog2/graylog-plugin-enterprise/pull/11084