Merge branch '75-gitlab-13-8-0' into 'master'

feat(gitlab): upgrade Gitlab to 13.8.0

Summary

Upgrade Gitlab to application version 13.8.0 chart version 4.8.0
Remove image pull secrets from the helmrelease template because they are now in the gitlab package code.

Closes #75

See merge request platform-one/big-bang/bigbang!214