test: only use mariadb 10.5.15 (#1121)

We use `10.5.15` in production; we should therefore target this for our tests, not two older versions.

Author: tarrow

.github/workflows/composer.test.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        databaseImageName: [ 'mariadb:10.5.9', 'mariadb:10.5.12' ]
+        databaseImageName: [ 'mariadb:10.5.15' ]
 
     runs-on: ubuntu-latest
     steps: