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/kernel/mempool_options.h

@@ -13,7 +13,7 @@
 #include <cstdint>
 #include <optional>
 
-class CMainSignals;
+class ValidationSignals;
 
 /** Default for -maxmempool, maximum megabytes of mempool memory usage */
 static constexpr unsigned int DEFAULT_MAX_MEMPOOL_SIZE_MB{300};
@@ -59,7 +59,7 @@ struct MemPoolOptions {
     bool persist_v1_dat{DEFAULT_PERSIST_V1_DAT};
     MemPoolLimits limits{};
 
-    CMainSignals* signals{nullptr};
+    ValidationSignals* signals{nullptr};
 };
 } // namespace kernel