Change MixedModeAlgorithm default to TCP. Closes #7779.

MixedModeAlgorithm::Proportional will throttle TCP connections when utp
is in use and users is expecting maximum speed no matter what, so now
disable the throttling.

src/base/bittorrent/session.cpp

@@ -304,7 +304,7 @@ Session::Session(QObject *parent)
     , m_btProtocol(BITTORRENT_SESSION_KEY("BTProtocol"), BTProtocol::Both
         , clampValue(BTProtocol::Both, BTProtocol::UTP))
     , m_isUTPRateLimited(BITTORRENT_SESSION_KEY("uTPRateLimited"), true)
-    , m_utpMixedMode(BITTORRENT_SESSION_KEY("uTPMixedMode"), MixedModeAlgorithm::Proportional
+    , m_utpMixedMode(BITTORRENT_SESSION_KEY("uTPMixedMode"), MixedModeAlgorithm::TCP
         , clampValue(MixedModeAlgorithm::TCP, MixedModeAlgorithm::Proportional))
     , m_multiConnectionsPerIpEnabled(BITTORRENT_SESSION_KEY("MultiConnectionsPerIp"), false)
     , m_isAddTrackersEnabled(BITTORRENT_SESSION_KEY("AddTrackersEnabled"), false)