Make label comparison case insensitive.

[luketomlinson]

Describe your issue

The comparison logic for when an issue was labeled stale should not be case sensitive.

[C0ZEN]

Why not use

stale/src/functions/is-labeled.ts

Lines 24 to 26 in 678bfc7

	function cleanLabel(label: Readonly<string>): CleanLabel {
	return deburr(label.toLowerCase());
	}

@luketomlinson