chore: Renames strings to auto-merge

Description

This MR attempts to change over the strings (not the command flag!) that mention any variant of MWPS or "merge when pipeline succeeds." Have I done it right? Will this look OK in context? I'm not sure yet; it needs testing by another person.

Related Issues

• Related to Plan for changes from 'auto_merge_labels_mr_wid... (technical-writing#782)
• Related to Replace merge strategy labels with `Auto-merge`... (gitlab#359057 - closed)
• Related to Add --auto-merge, make --when-pipeline-succeeds... (#1353)

How has this been tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gapchore