Lock MariaDB version to 10.4.12.

Author: bgrainger

azure-pipelines.yml

@@ -206,7 +206,7 @@ jobs:
         image: 'mariadb:10.3'
         unsupportedFeatures: 'Ed25519,Json,Sha256Password,CachingSha2Password,RoundDateTime,UuidToBin'
       'MariaDB 10.4':
-        image: 'mariadb:10.4'
+        image: 'mariadb:10.4.12'
         unsupportedFeatures: 'Json,Sha256Password,CachingSha2Password,RoundDateTime,UuidToBin'
   steps:
   - template: '.ci/integration-tests-steps.yml'