Lock MariaDB to bionic image.

(cherry picked from commit 185d53d)

Author: bgrainger

azure-pipelines.yml

@@ -203,7 +203,7 @@ jobs:
         image: 'mariadb:10.2'
         unsupportedFeatures: 'Ed25519,Json,Sha256Password,CachingSha2Password,RoundDateTime,UuidToBin'
       'MariaDB 10.3':
-        image: 'mariadb:10.3'
+        image: 'mariadb:10.3-bionic'
         unsupportedFeatures: 'Ed25519,Json,Sha256Password,CachingSha2Password,RoundDateTime,UuidToBin'
       'MariaDB 10.4':
         image: 'mariadb:10.4.12'