go.mod: Downgrade back to cty 1.12

We inadvertently incorporated the new minor release of cty into the 1.4
branch, and that's introduced some more refined handling of unknown values
that is too much of a change to introduce in a patch release.

Therefore this reverts back to the previous minor release for the v1.4
series, and then we'll separately get the main branch ready to work
correctly with the new cty before Terraform v1.5.

This reverts just the upgrade and the corresponding test changes from
PR #32775, while retaining the HCL upgrade and the new test case it
introduced for that bug it was trying to fix. That new test is still
passing so it seems that the cty upgrade is not crucial to that fix.

go.mod

@@ -170,7 +170,7 @@ require (
  github.com/spf13/cast v1.5.0 // indirect
  github.com/spf13/pflag v1.0.5 // indirect
  github.com/stretchr/objx v0.5.0 // indirect
- github.com/ulikunitz/xz v0.5.8 // indirect
+ github.com/ulikunitz/xz v0.5.10 // indirect
  github.com/vmihailenco/msgpack/v4 v4.3.12 // indirect
  github.com/vmihailenco/tagparser v0.1.1 // indirect
  go.opencensus.io v0.23.0 // indirect