This commit adds the upgrade guide from version
5.4 to 5.5.
Also, it removes all previous OSS guides not related
to version 5.5.
The guide includes the required Raft-related
information.
NOTE: The content of the guide must be further
verified closer to the release. I'm making
these updates now to avoid errors and warnings
related to outdated upgrade guides in other PRs,
and to include the Raft information.
Closesscylladb/scylladb#16350
ff2457157d