Draft: Configure gbp.conf correctly following DEP-14 and using pristine-tar

Done following the example set in https://salsa.debian.org/debian/dh-make/-/blob/master/lib/debian/gbp.conf.ex

This is marked as draft, as before any new development, the uploads that were never committed to git needs to be imported first (!2) and also the branch debian/unstable should be renamed to debian/latest before being able to finalize this MR by retargeting the correct branch name.