Change how MRs are requested from API
We've found that with current approach of requesting MRs (adding all MR IIDs to the URL query and paginating over results) we may get a 502 Bad Gateway
response from GitLab.
This MR changes the way how MRs are listed. For now on GitLab Changelog will ask for a given number of MRs (25 by default and non-configurable) using the same amount of MR IIDs.