Don't run the downtime check on stable branches, or tags/master

The way the check currently works, it runs on the diff between your
branch and the latest commit in gitlab on master.

This doesn't give a good representation of the changes migrations for
stable branches, tags, and master itself.

.gitlab-ci.yml

@@ -201,7 +201,13 @@ rake config_lint: *exec
 rake brakeman: *exec
 rake flay: *exec
 license_finder: *exec
-rake downtime_check: *exec
+rake downtime_check:
+  <<: *exec
+  except:
+    - master
+    - tags
+    - /^[\d-]+-stable(-ee)?$/
+
 rake ee_compat_check:
   <<: *exec
   only: