[Task] Block pull requests if pull-request template checkboxes are not met and if last commit is not signed

## Describe the feature
It would be nice if something like mergify could be used to block pull requests if the PR-template checkboxes are not filled. Something like this:
https://docs.mergify.com/examples/#matching-pull-requests-with-task-lists

There is also a way to block if the last commit is not signed.

## Any other information?
Nope.

Start by reviewing the repository's pull-request automation configuration and the linked Mergify task-list example. Define how pull requests should be blocked when template checkboxes are incomplete or the latest commit is unsigned, then verify both conditions through pull-request checks.

Written by the indexing model from the issue text.