Make markdown file check case insensitive

[ernilambar]

Check: File_Type_Check

In check_unexpected_markdown_files() check, "readme.md" was flagged which is not correct. This PR makes the file name check case insensitive to reduce false positives.

[github-actions]

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Co-authored-by: ernilambar <nilambar@git.wordpress.org>
Co-authored-by: davidperezgar <davidperez@git.wordpress.org>
Co-authored-by: frantorres <frantorres@git.wordpress.org>

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