chore: Pinning chroma-go ahead of major new release

[tazarov]

I'm making a major overhaul of the chroma-go package, so I need to pin the version until the upgrade is done, at which point I'll create a PR for any breaking changes.

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
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• Describes the source of new concepts.
• References existing implementations as appropriate.
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[tazarov]

@tmc, thanks for this. I'll make another PR once we do the refactoring to bring things up to par.