minor #15465 [Logger] Delete monolog_regex_based_excludes.rst (ThomasLandauer)

This PR was submitted for the 5.3 branch but it was merged into the 5.2 branch instead.

Discussion
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[Logger] Delete monolog_regex_based_excludes.rst

Since `excluded_404s` is deprecated in Monolog, I think this page can be deleted. Everything is said on https://symfony.com/doc/current/logging/monolog_exclude_http_codes.html

Commits
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1c7613b Delete monolog_regex_based_excludes.rst

Author: javiereguiluz