When a project approval rule is created that requires N approvers in the
project (also known as an `any_approver` rule), `MergeService` would
always fail with a "Name has already been taken" validation error.

This causes `MergeWorker` to run again and show different error messages
to the user. If squashing were enabled, users would see, "Merge failed:
Failed to squash. Should be done manually.. Please try again."
Otherwise, users would see, "Merge failed: Merge request is not
mergeable". In both cases, the merge was actually successful.

This error occurred because:

1. When a merge request is created, `MergeRequests::CreateService` syncs
some project approval rules (e.g. report types) to the list of merge
request approvals.

2. If only the `any_approver` rule is present,
`ApprovalRules::FinalizeService` will attempt to create a new merge
request approval rule since
`merge_request.approval_rules.regular.exists?` is `false`..

To fix this, we don't copy over the merge request rule if one by the
same name already exists.

Closes https://gitlab.com/gitlab-org/gitlab/issues/32477