[bugfix] self-referencing collection pages for status replies

[NyaaaWhatsUpDoc]

Description

• updates the paging package to use "slices" instead of the old experimental slices package
• updates StatusRepliesGet() processor function to use new collection and paging code
• updates GetStatusChildren() to be much simpler and return in case of reply fetching errors
• returns OrderedCollection instead of Collection from replies endpoint as our paging only works with an order ...
• updates the /replies endpoint to follow more AP compliant logic, this should be fine even with our compliance "guarantee" since this is an AP endpoint and not a client-side one

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 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.

[NyaaaWhatsUpDoc]

the new AP endpoint works as expected with other gotosocial instances, mastodon has issues though due to HTTP signatures and the fact the endpoint requires query parameters, so unrelated.

[tsmethurst]

Good stuff! Few comments but nothing breaking I believe except for the JSON type comment (should be activitystreams content-type)