[main] Jenkinsfile: update to use Ubuntu 22.04 agents

[thaJeztah]

• relates to [main] Makefile: set default branch when initializing git #366

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

OK; looks like this works, except for armhf; looks like we don't have machines labeled with that 🤔

[StefanScherer]

@thaJeztah Oh, just saw the comments. So is armhf still a thing in CI? Sorry, I removed them for newer Ubuntu at some point to make the packer builds a bit simpler, maybe I misunderstood something in other pipelines that it's no longer needed. 😬 Let me know if this is a blocker.