Drop support of Qt 5

Also remove usage of some deprecated stuff.

PR #19338.

src/base/bittorrent/tracker.cpp

@@ -140,11 +140,7 @@ namespace BitTorrent
         return !(left == right);
     }
 
-#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
     std::size_t qHash(const Peer &key, const std::size_t seed)
-#else
-    uint qHash(const Peer &key, const uint seed)
-#endif
     {
         return qHash(key.uniqueID(), seed);
     }