Feature request: Sign-off the Git commit

[hrj]

Background

This will help repositories which have a DCO check, for example in this repo

Of course, Scala Steward isn't a legal entity and the commits are very small anyway, so it doesn't really matter from a legal POV. The sign-off will only make the process of merging the PR smoother.

[fthomas]

I don't mind if Scala Steward does this. Dependabot does it too.

It is as easy as adding --signoff to this line.