MDEV-33750: Rename variables which are mysql_ to mariadb_ in Debian directory

[illuusio]

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• [x] The Jira issue number for this PR is: MDEV-33750

Description

This more about branding PR. As in many places when reading Debian scripts MariaDB is mentioned as MySQL (mysql_). As MariaDB is something else than MySQL (besides being mostly compatible upgrade path) it should be proudly be just MariaDB named variables in debian/ directory and get rid of other naming. There is also couple other changes under debian/ to brand this more.

Release Notes

This does not affect to release notes. It's more mental thing.

How can this PR be tested?

Build branch with debian/deb-autobake.sh and then install packages nothing should be changed or check Salsa-CI run https://salsa.debian.org/illuusio/mariadb-server/-/pipelines/672483 which fails badly (which are be taken care of in other PR) but one can see that it works as expected.

Basing the PR against the correct MariaDB version

• [x] This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• [x] I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• [x] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

[ottok]

This makes sense, just wondering why target stable 10.11 branch? Perhaps this should go in 11.4 or 11.5 instead?

[LinuxJedi]

I agree with @ottok, I don't seen any obvious problems in a quick scan of this PR, but I wonder why 10.11 was chosen instead of either 10.5 or 11.6 (with 11.5 as a sub for now).

[illuusio]

Ok I'll rebase this to 11.5. I just thought that rename won't be just big deal but it's no problem to rebase.

[CLAassistant]

All committers have signed the CLA.

[illuusio]

Ok now it's rebased to 11.5