[bugfix] Only allow boosting post from non-interaction-policy-aware instance if public or unlisted

[tsmethurst]

Description

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Fix a bug introduced by interaction policies in 0.17.0-rc1 where it was possible to try to boost a status from an instance that doesn't know about interaction policies, even when status should normally not be boostable.

Checklist

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