Error 405 for unsupported keyset pagination calls
What does this MR do?
Small change to respond with HTTP/405 instead of a HTTP/501 for requests that are not supported. In this case, we only support keyset pagination for the Projects API endpoint for one sorting order at the moment. Other cases should hit a HTTP/405.
This is per the status code documentation: https://docs.gitlab.com/ee/api/README.html#status-codes
The feature hasn't been released/deployed yet and is still behind a feature flag. Documentation is not yet merged either.