Mono repo -> Master (#22553)

Combines the following repositories into one:

https://github.com/mattermost/mattermost-server
https://github.com/mattermost/mattermost-webapp
https://github.com/mattermost/focalboard
https://github.com/mattermost/mattermost-plugin-playbooks

.gitlab-ci.yml

@@ -7,7 +7,7 @@ stages:
 
 include:
   - project: mattermost/ci/mattermost-server
-    ref: master
+    ref: monorepo-testing
     file: private.yml
 
 variables: