feat!: cleanup references of --compilers

[samuelhnrq]

PR Checklist

• Addresses an existing open issue: Fixes 🚀 Feature: Remove the long-deprecated --compilers option #5242
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Removes the last references from the code about --compilers there are still a lot of references in change logs and in documentation but was not sure if those should be removed or not.

[mark-wiemer]

Approved but not merging, waiting for Mocha 12 (#5357)

[mark-wiemer]

noting maybe we should use chore! instead to emphasize that this isn't a new feature in Mocha really

[JoshuaKGoldberg]

Technically this does change the user-facing behavior of Mocha. fix!, then? 😄

[mark-wiemer]

haha it's bikeshedding at this point, feat! is fine by me

[mark-wiemer]

tests are now failing

[mark-wiemer]

Tests passed locally after resolving merge conflicts :) will be merging this one tmrw @JoshuaKGoldberg

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.68%. Comparing base (ca4af43) to head (05376fc).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5403      +/-   ##
==========================================
- Coverage   93.73%   93.68%   -0.05%     
==========================================
  Files          57       57              
  Lines        4391     4389       -2     
  Branches      850      849       -1     
==========================================
- Hits         4116     4112       -4     
- Misses        275      277       +2     

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