This PR adds the 5.2-5.4 upgrade guide.
In addition, it removes the redundant upgrade guide from 5.2 to 5.3 (as 5.3 was skipped), as well as some mentions of version 5.3.
This PR must be backported to branch-5.4.
Closesscylladb/scylladb#15880
* github.com:scylladb/scylladb:
doc: add the upgrade guide from 5.2 to 5.4
doc: remove version "5.3" from the docs
doc: remove the 5.2-to-5.3 upgrade guide
(cherry picked from commit 74f68a472f)