Reword Gitlab to GitLab

I saw that inside the handbook pages we use GitLab and Gitlab (notice the capital L difference) interchangeably.

We should instead always use GitLab.

Proposal

Change all the occurences of Gitlab to GitLab (this will be split into multiple MRs to avoid too many changes per MR)

Add a test to make sure that we spot the usage of Gitlab in the future.