[chore] bump go structr cache version -> v0.6.0

[NyaaaWhatsUpDoc]

Description

• updates go-structr to v0.6.0, which made some incompatible API changes that this PR fixes. this latest version adds the Queue{} type to the structr package which allows (in the future) for indexable queues.
• updates all the cache.GTS{} caches to be struct value types instead of a mixture of values + ptrs
• adds structr.Cache{} wrapping type to simplify updating with the incompatible API changes + allows for much simpler loading of multiple IDs from caches

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

Looks good to me! Just the one question about the index map :) And the accidental test output commit ;)

[tsmethurst]

Squerge when you're happy!

[tsmethurst]

(Just merged main into this to fix the docs fail)