gitlab: prefill MR template with first multiline commit message

Resolve situation for one commit MR, with description inside.

Acked-By: Mike Blumenkrantz <michae
l.blumenkrantz@gmail.com>

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23346>

.gitlab/merge_request_templates/Default.md

@@ -1,7 +1,4 @@
 ### What does this MR do and why?
 <!-- Describe in detail what your merge request does and why. -->
 
-### This MR was tested:
- - [ ] manually tested on my development machine(s) with the following:
- - [ ] repeatedly tested since there is a risk of introducing flakes
- - [ ] not applicable
+%{first_multiline_commit}