gitlab: Add merge request template for backports for 8.8

.gitlab/merge_request_templates/backport-for-8.8.md

+Thank you for help in maintaining GHC's stable branch!
+
+Please take a few moments to verify the following:
+
+ * [ ] the issue that this backport fixes is milestoned for the target release.
+
+If you have any questions don't hesitate to open your merge request and inquire
+in a comment.
+
+/label ~backport
+/milestone %8.8.1