Add info on upgrading to Runner 10
What does this MR do?
Add information how to update to Runner 10+
Closes https://gitlab.com/gitlab-org/gitlab-runner/issues/2784
Edited by Achilleas Pipinellis
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Add information how to update to Runner 10+
Closes https://gitlab.com/gitlab-org/gitlab-runner/issues/2784