[bugfix] Always serialize orderedItems as array in OrderedCollection

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

This pull request implements a workaround for an issue where orderedItems in a featured/pinned collection would be serialized as a string (instead of a string array) if there was only one entry. This was causing issues with other implementations that assume that orderedItems will always be an array (which is apparently the correct behavior).

See mastodon/mastodon#24225 and go-fed/activity#139

Note: while this code gets us moving for now, it is far from a perfect solution because a) it's confusing and b) it's inefficient. We should definitely pull this code out if/when we get it fixed in go-fed.

Checklist

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If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• 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 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.