Rename `ATMTWPS` to `MTWPS` in code

The following discussion from !14097 (merged) should be addressed:

• @filipa started a discussion: (+5 comments)

  what does ATMTWPS mean? 🤔

As discussed in the thread linked above, we've been using the acronym ATMTWPS for "Add To Merge Train When Pipeline Succeeds". This is pretty unwieldy; let's use MTWPS instead ("Merge Train When Pipeline Succeeds").