CI: Update to the latest Buster release.

The workflow is configured to use an old buster release and the
latest (in case default versions in old OS releases fail to run
Mu). So the "old buster" image has been changed to an older one.

.github/workflows/test.yml

@@ -100,7 +100,7 @@ jobs:
  runs-on: ubuntu-latest
  strategy:
  matrix:
- docker-tag: ['stretch-2018-03-13', 'buster-2021-05-28', 'buster-legacy-2022-04-07']
+ docker-tag: ['stretch-2018-03-13', 'buster-2021-03-25', 'buster-legacy-2023-05-03']
  fail-fast: false
  services:
  rpios: