This commit replaces the link to the OSS-only page
(the 5.2-to-5.4 upgrade guide not present in
the Enterprise docs) on the Raft page.
While providing the link to the specific upgrade
guide is more user-friendly, it causes build failures
of the Enterprise documentation. I've replaced
it with the link to the general Upgrade section.
The ".. only:: opensource" directive used to wrap
the OSS-only content correctly excludes the content
form the Enterprise docs - but it doesn't prevent
build warnings.
This commit must be backported to branch-5.4 to
prevent errors in all versions.
Closesscylladb/scylladb#16176