gitlab: Use ListMergeRequestDiff instead of GetMergeRequestChanges

[chmouel]

since gitlab deprecated it as documented here:

https://docs.gitlab.com/ee/api/merge_requests.html#get-single-merge-request-changes

pkg/provider/gitlab/gitlab.go:315:24: SA1019: v.Client.MergeRequests.GetMergeRequestChanges is deprecated: This endpoint has been replaced by MergeRequestsService.ListMergeRequestDiffs() (staticcheck)
		mrchanges, _, err := v.Client.MergeRequests.GetMergeRequestChanges(v.sourceProjectID, runevent.PullRequestNumber, &gitlab.GetMergeRequestChangesOptions{}