Update mariadb-secure-installation to take into account unix_socket authentication

This script (usually referred to as mysql_secure_installation) is heavily used based on existing guidance/tutorials.

However, the flow makes no sense when Unix socket authentication is applied (as it is by default since 10.4). It's a confusing experience for users and should be fixed.

Suggestions:

If unix_socket_authentication is enabled:
1) Clarify that running the script is mostly unnecessary, perhaps using the text at the top of https://mariadb.com/kb/en/mysql_secure_installation/, and asking whether they are sure they wish to continue.

2) Remove the question "Switch to unix_socket authentication [Y/n]"

3) Remove the question "Change the root password? [Y/n]"

See also discussion on https://mariadb.zulipchat.com/#narrow/stream/118759-general/topic/'mariadb-secure-installation'.20script.20-.20root.20access where 2) and 3) were suggested by Red Hat's Michal Schorm, as well as MDEV-26593, which proposed solving the issue by removing the script entirely.