Add support for running openHAB with Java 21

[wborn]

Today Java 21 will be launched. :rocket: This is a LTS release, so we should also support using it with openHAB.

We can use this issue to keep track of what still needs to be fixed or investigated (similar to #1316).

There's also a launch live stream: https://dev.java/community/java-21-launch/

TODO:

• [x] Upgrade Netty to 4.1.99 or newer to prevent JVM crashes (see 4.1.99 release notes) (https://github.com/openhab/openhab-core/pull/3824)
• [ ] Update Java compatibility check in karaf and karaf.bat