Update builder tiers to match PEP 11

[StanFromIreland]

This makes the tiers consistent with those specified by PEP 11 (unfortunately some changes are not quite correct for older branches, as tiers aren't specific per branch). Also, technically it isn't a complete update as RISC-V is left to #791 (since that includes an additional change from unstable to stable).

[StanFromIreland]

Consistent with:

buildmaster-config/master/custom/factories.py

Lines 984 to 987 in e78921c

	# The non-debug WASI buildbot is meant for 3.11 and 3.12 only.
	# Don't use it on PRs; it's tier 3 only and getting it to
	# work on PRs against `main` is too much work.
	self.branches = {BRANCHES[3, 11], BRANCHES[3, 12]}

[vstinner]

Oh, I merged your RISC-V change which introduced a conflict in this change. I reviewed the RISC-V first, but it would be be better to merge this change first since it's way later. Well, too late :-)

[StanFromIreland]

No worries, there would be some conflicts in either order. Fixed now. :-)