Fixes https://github.com/scylladb/scylladb/issues/14097
This commit removes support for Ubuntu 18 from
platform support for ScyllaDB Enterprise 2023.1.
The update is in sync with the change made for
ScyllaDB 5.2.
This commit must be backported to branch-5.2 and
branch-5.3.
Closes #14118
(cherry picked from commit b7022cd74e)