Move all merge request documents over to a new index

Proposal

Move all of the merge request documents over to a new index. This may be done over a few MRs, but they must be merged together through a parent MR and all should be attached to this issue.

• Create a new mapping config to allow read/write to a separate index for a new merge requests index (example: Issue mapping config file)
• Add a migration that pauses indexing and moves the merge requests over to the new index
• Ensure reads and writes for merge requests are now coming from the new index
• Add a migration that deletes the merge requests documents from the existing index