ci: run applicable jobs on the Clever CI

XS has 1 CPU and 2GB of RAM. That should do — since we have a lot of parallel tests, let's pick the cheapest runner for most tests. Except the first two runs that are most likely to fail, let's use a slightly bigger M instance with 4 cores and 8GB of RAM. And given that Clever scales quite nicely, let's fold the Upcoming stage into the other compatibility tests, so everything runs concurrently.

While at it, I suppressed gathering of coverage files for failed jobs. Typically, such numbers will be bogus, and doing so prevents a re-run from automatically generating the correct numbers.