Minor: change template header level

What is the new feature

See title. Simply change the level of the last header (mandatory steps before merge).

How is it implemented

The template looks awkward currently.

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• New feature is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Reviewed by a maintainer
• Incorporate review suggestions

You are not supposed to merge this request by yourself, the maintainers of fortan-support take care of this action!