fix(Core/Misc): all GCC warnings (#3457)

src/server/game/Handlers/LFGHandler.cpp

@@ -373,7 +373,7 @@ void WorldSession::SendLfgUpdateParty(lfg::LfgUpdateData const& updateData)
     {
         case lfg::LFG_UPDATETYPE_ADDED_TO_QUEUE:                // Rolecheck Success
             queued = true;
-            // no break on purpose
+            [[fallthrough]];
         case lfg::LFG_UPDATETYPE_PROPOSAL_BEGIN:
             join = true;
             break;

src/server/game/Handlers/PetHandler.cpp

@@ -647,6 +647,7 @@ void WorldSession::HandlePetActionHelper(Unit* pet, uint64 guid1, uint16 spellid
                     if (pet->ToPet())
                         pet->ToPet()->ClearCastWhenWillAvailable();
                     pet->ClearInPetCombat();
+                    [[fallthrough]]; // TODO: Not sure whether the fallthrough was a mistake (forgetting a break) or intended. This should be double-checked.
 
                 case REACT_DEFENSIVE:                       //recovery
                 case REACT_AGGRESSIVE:                      //activete