[ticket/11951] Add MariaDB to supported DBMSes in requirement section.

PHPBB3-11951
[ticket/11951] Add MariaDB to platforms we test on.

PHPBB3-11951
Merge remote-tracking branch 'github-bantu/ticket/11951' into develop-olympus

* github-bantu/ticket/11951:
  [ticket/11951] Add MariaDB to platforms we test on.
  [ticket/11951] Add MariaDB to supported DBMSes in requirement section.