scripted-diff: Rename MainSignals to ValidationSignals

-BEGIN VERIFY SCRIPT-
s() { git grep -l "$1" src | xargs sed -i "s/$1/$2/g"; }

s 'CMainSignals'    'ValidationSignals'
s 'MainSignalsImpl' 'ValidationSignalsImpl'
-END VERIFY SCRIPT-

src/bitcoin-chainstate.cpp

@@ -74,7 +74,7 @@ int main(int argc, char* argv[])
     // Start the lightweight task scheduler thread
     scheduler.m_service_thread = std::thread(util::TraceThread, "scheduler", [&] { scheduler.serviceQueue(); });
 
-    CMainSignals validation_signals{scheduler};
+    ValidationSignals validation_signals{scheduler};
 
     // Gather some entropy once per minute.
     scheduler.scheduleEvery(RandAddPeriodic, std::chrono::minutes{1});