update go-cty@v1.15.0

[jbardin]

Update go-cty to v1.15.0

• function/stdlib: The element function now accepts negative indices, extending the illusion of an infinitely-long list into the negative direction too.
• cty: The various "transform" functions were previously incorrectly propagating marks from a parent object down to attribute values when calling the caller-provided transform functions. The marks will now no longer be propagated downwards, which is consistent with the treatment of collection and tuple elements. If your transform function needs to take into account context about marks of upstream containers then you can maintain a stack of active marks in your Transformer implementation, pushing in Enter and popping in Exit. (#185)

Closes #35424
Closes #15582

[apparentlymart]

The first change mentioned in the changelog would close #15582, I think.

[jbardin]

Ah, there its is! thanks!

[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]

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