[ticket/11920] Pin MariaDB repository to avoid conflicts with Ubuntu MySQL.

PHPBB3-11920
Merge remote-tracking branch 'bantu/ticket/11920' into develop-olympus

* bantu/ticket/11920:
  [ticket/11920] Pin MariaDB repository to avoid conflicts with Ubuntu MySQL.
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/11920] Pin MariaDB repository to avoid conflicts with Ubuntu MySQL.