Make forward pagination cursor strings different from backward

[dullbananas]

Requirements

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Is your proposal related to a problem?

Forgetting to set page_back to true could be a very likely mistake that's hard to debug, and needing to set page_back makes using the api require more complicated code

Describe the solution you'd like.

Remove page_back from the api, and make paginated responses have a previous page cursor that has an underscore added at the beginning

Should be implemented in or after #5424

Describe alternatives you've considered.

None

Additional context

No response