.github/workflows: update actions to use latest versions

The logs for the CI builds are indicating that older actions based upon Node 20
are deprecated. Update the following versions:

  actions/checkout@v4 -> actions/checkout@v7
  actions/download-artifact@v4 -> actions/download-artifact@v8
  actions/upload-artifact@v4 -> actions/upload-artifact@v7
  docker/build-push-action@v5 -> docker/build-push-action@v7
  docker/login-action@v3 -> docker/login-action@v4
  docker/metadata-action@v5 -> docker/metadata-action@v6

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>