Require PHP 8.4 and use the new DOM API

[janedbal]

Summary

• Bump the minimum PHP version to 8.4.
• Replace DOMDocument::loadXML / loadHTML with Dom\XMLDocument::createFromString and Dom\HTMLDocument::createFromString.
• Keep the API surface: XmlLoader::loadXml(), XmlLoader::loadHtml(), and XmlException wrapping LibXMLError stay the same.
• Keep the safety guarantees: LIBXML_NONET | LIBXML_NOBLANKS for XML, empty-input rejection, and DOCTYPE rejection.
• Run CI on PHP 8.4 and 8.5 only.

Breaking changes (v4)

• loadXml() returns Dom\XMLDocument and loadHtml() returns Dom\HTMLDocument. These classes do not extend DOMDocument.
• The new DOM classes follow the DOM spec. For example, nodeValue on an element returns null; use textContent instead.
• loadHtml() parses with the HTML5 parser (Lexbor). The HTML5 parser recovers from malformed markup, so loadHtml() throws only on empty input or a hard parser failure. The old HTML 4 parser had almost the same failure behavior.

Notes

• The new API does not force LIBXML_NONET by itself. The loader still passes it explicitly.
• Parse-error detail still comes from libxml_get_last_error(), so XmlException::getError() keeps line and column information.
• Tests assert the entity-expansion errors with wildcard patterns, because libxml changed those messages in 2.11.