Optimize used hetzner machine classes

Currently the instance runners use hetzners runner classes: cpx21, cpx31, cpx41.

c(p)x21 as the minimal available (or at least auto-selected) runner is sensible, as most common build environments around java, nodejs, docker/dind may require slightly more than the 2GB RAM of the c(p)x11 series of runners. It's more stable to use at least 21-series as default runners. When going up with resources in 31, 41, and 51 series machines it might make sense to use the more powerful (p)erformance variants. If you opt-in to use those you probably not only need more RAM but also more powerful computational power.

When using tags, always remember that you might want to pin the resource class as otherwise machines with similar capabilities but much higher cost and resources than required might be selected, as they can't be prioritized.