Replace patchwork/utf8 with symfony/polyfill

patchwork/utf8 package is abandoned and no longer maintained. The tchwork/utf8 repository has been archived by the author.
The author suggests using the symfony/polyfill-mbstring or symfony/string package instead. See https://packagist.org/packages/patchwork/utf8

patchwork/utf8 is used in /includes/utf/utf_tools.php to provide UTF-8 compatibility layer and can be replaced with several parts of symfony/polyfill:
Patchwork\Utf8\Bootup::initUtf8Encode() -> symfony/polyfill-php72 for utf8_encode()/utf8_decode() functions;
Patchwork\Utf8\Bootup::initMbstring() -> symfony/polyfill-mbstring for mb_*() functions;
Patchwork\Utf8\Bootup::initIntl() -> symfony/polyfill-intl-normalizer for Normalizer::normalize()/Normalizer::isNormalized() functions.

Currently all 3 packages of symfony/polyfill-mbstring, symfony/polyfill-intl-normalizer, symfony/polyfill-php72 are already installed in phpBB as dependencies of other symfony packages.