build: go get github.com/hashicorp/hcl/v2@v2.12.0

[apparentlymart]

This tagged release is a superset of the specific commit we had previously selected, adding in just an improved error message for inconsistent result types in a conditional expression.

The error message for #30562 would become the following after merging this:

╷
│ Error: Inconsistent conditional result types
│ 
│   on fun-with-tuples.tf line 20, in output "test":
│   20:   value = true ? { foo = { bar = 42 } } : { bar = 44 }
│ 
│ The true and false result expressions must have consistent types.
│ The 'true' value includes object attribute "foo", which is absent
│ in the 'false' value.
╵

This fixes #30562 and fixes #29226. (by improving the error message to be more specific, not by changing what is valid.)

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
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