Unit test against more versions of MariaDB

- 10.3, 10.4, 10.5, 10.6, 10.11 are the
  currently-in-support versions of MariaDB 10.X
- No version of MariaDB 11 has long term support, so test against
  the newest image in the 11 series.
- Also test against the "latest" docker image available. In the near
  term, this is likely to the same image as "11". However testing
  against "latest" will mean we test against 12 automatically when
  it is released.

.github/workflows/continuous-integration.yml

@@ -14,7 +14,7 @@ jobs:
         # Define jobs for all combinations of php, mariadb and extension, up to "include"
         # Tests will be performed for each combination
         php-version: ["5.4", "5.5", "5.6", "7.0", "7.1", "7.4"]
-        mariadb-version: ["10.3"]
+        mariadb-version: ["10.3", "10.4", "10.5", "10.6", "10.11", "11", "latest"]
         extension: ["pdo_mysql"]
         include:
           - php-version: "5.4"