fix(gitlab): s/PATCH/PUT on MR note modification API call. (#2690)

When trying to modifying an existing note for a merge request,
RenovateBot ended up with an `Error updating branch`, the API call
returning a `404 Not Found`.
After taking a look at the GitLab API documentation
(https://docs.gitlab.com/ee/api/notes.html#modify-existing-merge-request-note),
the right call for modifying a MR notes should be perform using the
`PUT` HTTP Method.

This has been discovered because I observed errors after commit
6577651f that modify all occurences of
`Pull request` (or `PR`) to `Merge Request` (or `MR`) of notes bodies
before updated it.