Test on Ruby 3.1

[cben]

stealing @kjetijor's commit 0781a9d from #537 (again, this time on master branch)
I'm still not sure where net/smtp actually gets require-d, but pretty certain it's not in kubeclient itself so a dev dependency sounds right.

Fixes #543 🎉

[cben]

2.5 run was just stuck, which I'm sometimes seeing recently. minitest starts, and then... nothing for 20min? (because I cancelled manually. saw 5hours in another run!)

Run options: --seed 40851

# Running:

Error: The operation was canceled.

re-running...

[cben]

2.5 stuck again. But OK, 3.1 and ruby-head (3.2-to-be) passed, merging.

I'll add test timeouts in another PR.