unicodeTrim: Fix PHP 8.2 deprecation

mbstring extension in PHP 8.2 deprecates `HTML-ENTITIES` encoding:
https://php.watch/versions/8.2/mbstring-qprint-base64-uuencode-html-entities-deprecated

However, there is no need to use it to the UTF-8 representation of NBSP, one can just directly use `\u{0a}` (or `\xc2\x0a` for PHP < 7.0).

Or, even better, we can enable `PCRE_UTF8` mode:

https://www.php.net/manual/en/reference.pcre.pattern.modifiers.php

It is supposed to be available since PHP 5.1:

https://www.phpbb.com/community/viewtopic.php?t=733515

Author: jtojnar

Mf2/Parser.php

@@ -111,9 +111,7 @@ function collapseWhitespace($str) {
 }
 
 function unicodeTrim($str) {
-	// this is cheating. TODO: find a better way if this causes any problems
-	$str = str_replace(mb_convert_encoding(' ', 'UTF-8', 'HTML-ENTITIES'), ' ', $str);
-	return preg_replace('/^\s+|\s+$/', '', $str);
+	return preg_replace('/^\s+|\s+$/u', '', $str);
 }
 
 /**