fix: detect legacy 'based on' notices in README.md

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: README.md additional content should ignore older "this package is based on" notices #2124
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

🎁

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.80%. Comparing base (5a03263) to head (e1ca36c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2125   +/-   ##
=======================================
  Coverage   99.80%   99.80%           
=======================================
  Files         127      127           
  Lines        5246     5246           
  Branches      371      371           
=======================================
  Hits         5236     5236           
  Misses         10       10           

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[github-actions]

🎉 This is included in version v2.24.2 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀