Add locked state to merge request API

It is impossible to use the merge request API to find merge requests in a "locked" state.

They are searchable using the same API without a filter, or with state=all, but get an error if trying to search with state=locked. I get the following error:

Net::HTTPBadRequest 400 Bad Request readbody=true

I understand the "locked" is a very short-lived, transitional state, but this issue is reproducible if trying to call the API from a custom hook which is triggered when accepting a merge request in the UI.