actions(tests): use mariadb 10.5 instead of 10.6

10.6.2 (latest docker build as of 2021-07-09) is a RC, not a stable release.
10.6.3 is out and is a stable release, however, no docker image is built for it yet.
10.6 should be added or replace 10.5 once a stable docker image is available.

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
       fail-fast: false
       matrix:
         php: [ 7.4, 8.0 ]
-        database: [ 'mariadb:10.2', 'mariadb:10.6', 'mysql:8' ]
+        database: [ 'mariadb:10.2', 'mariadb:10.5', 'mysql:8' ]
     services:
       database:
         image: ${{ matrix.database }}