Include first multiline commit in MR description for default template

What does this merge request do and why?

I often spend a long time writing my commit messages, only to spend an extra step copying and pasting these into the merge request description. Having the text automatically inserted would save time.

This uses %{first_multiline_commit} to include the first multiline commit in the MR description.

Docs: https://docs.gitlab.com/ee/user/project/description_templates.html

We did this same change in other projects:

• gitlab!118413 (merged)
• omnibus-gitlab!6692 (merged)

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.