ci: rework commit message template

Once mergify exposes co-authorship as a first-class attribute (see https://github.com/Mergifyio/mergify/discussions/5091), we should work on our commit message template again.

In particular, we have the following requirements:

1. There must not be any empty lines between any of the trailers for git interpret-trailers to work correctly.
2. Each trailer must be on a separate line.

This presents a bit of a challenge that I'd like to document here. For one, to save on characters in the commit message title, we want to link to the pull-request via a trailer, like so: Pull-request: #XYZ. This would be error-prone to do manually so this should done automatically in the template. As a result, we will always have at least one trailer!

To automatically close issues or link to related ones, we often use the Resolves: #XYZ or Related: #XYZ trailer. Here comes the tricky bit:

• Trailers need to be separated with a newline from the main message body, otherwise they are not considered trailers but just regular lines of the commit message.
• We do not always have a a Resolved or Related trailer.

Can we write a Jinja2 template that always correctly generates the trailers? I think we might have to create a separate section in our pull-request template for handwritten trailers. This section would be inserted without newlines in the commit message whereas the main ## Description section would be unconditionally separated with a newline from the trailers.

Start by reading the Jinja2 commit-message template and the pull-request template section described in the issue, then verify how git interpret-trailers treats blank lines and trailer placement. Done means the generated message always includes the pull-request trailer, keeps trailers on separate adjacent lines, and handles optional Resolves or Related trailers correctly after Mergify supports co-authorship.