Rename MWPS to Auto-merge in Features

Why is this change being made?

As part of gitlab-org&6687 (closed) we've simplified the MWPS/MT/MTWPS text on the Merge button to be Auto-merge. This reflects a simpler language and enforces a single pattern regardless of the things that will happen when merging. This sets the stage for a broader Auto-merge concept which would allow us to set things to merge when all requirements are satisfied instead of just the pipeline.

This change is active and in product, so this update reflects the required changes to features.yml and to other places where the MWPS text/acronym was used in long-lived content.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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  • The when to get approval handbook section explains the workflow in more detail
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