merge v7.4.0 for experimental

Hi, since I was looking at v7.4.0 for Ubuntu I wanted to share this MR to not accumulate too much churn after Debian is released. This would be a chance to discuss changes up-front and maybe even pass some new-queue hops now in -experimental.

Since we want to keep master branch (and unstable uploads) free for the coming release this is targeted against experimental.

Current salsa/debian/experimental can cleanly be merged vs debian/salsa/master so that is the start of this branch. Then next is the result of the gbp import. And then finally few patch drops and updates to debian/* that are needed to build v7.4.0.

FYI: I've opened pristine-tar !106 (closed) and upstream/latest !105 (closed) MRs that go along this MR.

P.S. I see we have killed the experimental branch since a while. So this goes against master, but really should go to -experimental if we'd branch it off current master again.