spec, go/types, types2: restore Go 1.20 unification when compiling for Go 1.20 [1.21 backport] #61930
Description
@dmitshur requested issue #61903 to be considered for backport to the next 1.21 minor release.
The fix needs to land on the main branch first (i.e., where Go 1.22 development happens), and then it can be backported to release-branch.go1.21. I'll move this to Go1.22 milestone and create a backport issue to track the fix for Go 1.21.1.
@gopherbot Please backport to Go 1.21 with the rationale @rsc provided:
[...] we should at least restore the old behavior when go.mod says "go 1.20".
Let's try to do that in the next point release.