Update MD_HEADER_REGEXP to require at least 4 equals signs

[Ayoub-Mabrouk]

updates the regular expression for validating the header in HISTORY.md. The new pattern enforces at least 4 consecutive equals signs (=) instead of any number of equals signs.

Changes:
Old RegExp: /^====*$/ (allowed any number of equals signs).
New RegExp: /^={4,}$/ (requires at least 4 equals signs).
Rationale:
This ensures a more consistent and readable header format in HISTORY.md

[UlisesGascon]

Hey @Ayoub-Mabrouk!

Thanks for this PR! While the readability is much better the performance has being decreased. I created this small POC to showcase the difference

[wesleytodd]

Also, we will be getting rid of this integration in the near future. I would not worry too much about the readability OR the performance for this. expressjs/discussions#293

EDIT: Oops, I did not mean to click the close button. That said, @Ayoub-Mabrouk I would recommend engaging in the thread I linked and maybe you can help us improve the history files for the entire project? Feel free to re-open if you would rather continue to engage on this one.