Fix fatal error that is thrown when the input string has an invalid encoding

[JulianVennen]

When the input license text has an invalid encoding a fatal error is thrown by the html to markdown converter:

PHP Fatal error:  Uncaught TypeError: preg_replace(): Argument #3 ($subject) must be of type array|string, null given in /web/vendor/league/html-to-markdown/src/Converter/TextConverter.php:24
Stack trace:
#0 /web/vendor/league/html-to-markdown/src/Converter/TextConverter.php(24): preg_replace()
#1 /web/vendor/league/html-to-markdown/src/HtmlConverter.php(222): League\HTMLToMarkdown\Converter\TextConverter->convert()
#2 /web/vendor/league/html-to-markdown/src/HtmlConverter.php(193): League\HTMLToMarkdown\HtmlConverter->convertToMarkdown()
#3 /web/vendor/league/html-to-markdown/src/HtmlConverter.php(188): League\HTMLToMarkdown\HtmlConverter->convertChildren()
#4 /web/vendor/league/html-to-markdown/src/HtmlConverter.php(188): League\HTMLToMarkdown\HtmlConverter->convertChildren()
#5 /web/vendor/league/html-to-markdown/src/HtmlConverter.php(188): League\HTMLToMarkdown\HtmlConverter->convertChildren()
#6 /web/vendor/league/html-to-markdown/src/HtmlConverter.php(188): League\HTMLToMarkdown\HtmlConverter->convertChildren()
#7 /web/vendor/league/html-to-markdown/src/HtmlConverter.php(188): League\HTMLToMarkdown\HtmlConverter->convertChildren()
#8 /web/vendor/league/html-to-markdown/src/HtmlConverter.php(99): League\HTMLToMarkdown\HtmlConverter->convertChildren()
#9 /web/vendor/aternos/licensee/src/TextTransformer/HtmlTransformer.php(26): League\HTMLToMarkdown\HtmlConverter->convert()
#10 /web/vendor/aternos/licensee/src/License/Text/LicenseText.php(106): Aternos\Licensee\TextTransformer\HtmlTransformer->transform()
#11 /web/vendor/aternos/licensee/src/License/Text/LicenseText.php(131): Aternos\Licensee\License\Text\LicenseText->getNormalizedContent()
#12 /web/vendor/aternos/licensee/src/Matcher/ExactMatcher.php(15): Aternos\Licensee\License\Text\LicenseText->getWordSet()
#13 /web/vendor/aternos/licensee/src/Matcher/Matcher.php(45): Aternos\Licensee\Matcher\ExactMatcher->match()
#14 /web/vendor/aternos/licensee/src/Matcher/Matcher.php(62): Aternos\Licensee\Matcher\Matcher->getAllMatches()
#15 /web/vendor/aternos/licensee/src/Licensee.php(80): Aternos\Licensee\Matcher\Matcher->getMatch()
#16 /web/src/Processor/MetaDataExtractor/MetaFile/LicenseFile.php(34): Aternos\Licensee\Licensee->findLicenseByContent()

This PR fixes that by converting the text to UTF-8 using mb_convert_encoding before applying the transformers. The mbstring extension has been added as a required dependency.