Rename MergeRequestsClosingIssues model/table

MergeRequestsClosingIssues model and related merge_requests_closing_issues used to contain records that associate a merge request with an issue. If arecord existed in this table, merging the MR would cause the issue to be closed when that happened (if enabled for the project). With &13366 (closed) we want to be able to add "related MRs" for an issue without it meaning that it will close the issue. We are going to use this same table for both things as we want those related merge requests to be listed together in the new work item development widget. For that reason we need to find a better name for this table