Relax perl client dependencies in mariadb-client (Closes: #1115678)

Part of Ubuntu's MIR for mariadb (LP: #2122095) and Debian bug #1115678 both point out the same thing: mariadb-client pulls in Perl tooling by default, even for users who only want the C client binaries. In particular, it still Depends on libconfig-inifiles-perl (universe) and Recommends a bunch of Perl DBI modules.

libconfig-inifiles-perl is a historical leftover. Debian once carried a mytop patch (added in commit 0d0f3ac0 on 2017-06-19) that added use Config::IniFiles; and made mytop read ~/.my.cnf through that module. That led to bug #875708 ("[mytop] missing libconfig-inifiles-perl"), which was fixed at the time by adding a hard dependency. Later that patch was dropped again in commit cb2a1609 on 2020-08-23, when upstream mytop became "good enough", and upstream never used Config::IniFiles in the first place. In the current 11.8.x tree there are no references to Config::IniFiles, so there is nothing left that actually needs libconfig-inifiles-perl.

This MR does three small, targeted things:

• Drop libconfig-inifiles-perl from mariadb-client's Depends, since nothing in the current source uses it.
• Move the Perl DBI stack (libdbi-perl, libdbd-mariadb-perl | libdbd-mysql-perl, libterm-readkey-perl) from Recommends to Suggests, because these are only needed for the optional Perl tools (mytop, innotop, mariadb-report), not for mariadb, mysqldump, etc.
• Extend debian/mariadb-client.README.Debian with a short note so that users who run those Perl tools and hit Can't locate DBI.pm are pointed at the right packages to install.

This keeps the Perl clients available and documented, but removes a dead hard dependency and stops pulling the Perl stack into the default mariadb-client footprint, which is exactly what the MIR and Debian #1115678 are asking for.