chore: remove old Qt 5.12 code, update docs and FreeBSD runner (#5396)

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/util/QStringHash.hpp

@@ -4,8 +4,6 @@
 #include <QHash>
 #include <QString>
 
-#include <functional>
-
 namespace boost {
 
 template <>
@@ -17,17 +15,3 @@ struct hash<QString> {
 };
 
 }  // namespace boost
-
-namespace std {
-
-#if QT_VERSION < QT_VERSION_CHECK(5, 14, 0)
-template <>
-struct hash<QString> {
-    std::size_t operator()(const QString &s) const
-    {
-        return qHash(s);
-    }
-};
-#endif
-
-}  // namespace std