Test: remove auto_merge_labels_mr_widget in tests

Resolves #424829 (closed)

What does this MR do and why?

The auto_merge_labels_mr_widget feature flag has been removed and we are getting warnings about it in tests.

This MR, removes the usage of the FF in tests. One of the tests also needed naming updates due to MWPS changing to auto-merge

There are two quarantined tests which were updated so they were passing locally.

How to set up and validate locally

rspec ./spec/features/merge_request/user_sets_to_auto_merge_spec.rb

MR acceptance checklist

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