Add notes for spotbugs jdk21 support

What does this MR do and why?

Update Spotbugs documentation to describe how users can enable JDK21 support.

Version 6 of the Spotbugs analyzer introduces JDK21 support but also removes JDK11, which is a breaking change. Since this breaking change was not approved, we cannot update the stable CI template to use version 6 by default. This documentation serves to guide users on how to enable JDK21 support by manually pinning to version 6.

More details in this thread.

Related issues

Update spotbugs major version in SAST templates (#517169 - closed)

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