Bump mariadb CI images + mysql unpin (#2739)

With MariaDB 10.3 EOL it seems prudent to include
the latest 10.11 LTS release in the CI tests.

To catch #1664 sort of issues prior to release, add
a container image test for mariadb that contains the
finished pre-release code changes (and container changes)
to ensure that compatibilty is maintianed. When
server code hits the branch associated with this tag
it is considered finished by its server developers and
has passes CI so no regressions are expected, but
it wouldn't hurt to have CI in other systems testing
it too.

Since MySQL has fixed the regression that caused #1664
this puts it back to their 8.0 release tag.

Append the :z tag to the docker compose volume mounts
so on selinux systems enough permissions occur for the
volume to be able to read the volume contents.

.github/workflows/sqlx.yml

@@ -467,7 +467,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        mariadb: [10_6, 10_3]
+        mariadb: [verylatest, 10_11, 10_4]
         runtime: [async-std, tokio]
         tls: [native-tls, rustls, none]
     needs: check