Release notes showing Debian 11.x support (not due until 7.1)

We rolled back a change because https://issues.couchbase.com/browse/DOC-9080 went a bit too early.

Looks like the release notes were not rolled back too, so they're showing that Debian 11.x is now a supported platform. 

That section needs rolling back:

https://docs.couchbase.com/server/current/release-notes/relnotes.html#supported-platforms-700