Make ObjectMgrs _mailId and _hiPetNumber atomic (#19129)

- They can be accessed from multiple threads simultaneously

src/server/game/Globals/ObjectMgr.cpp

@@ -223,7 +223,6 @@ bool SpellClickInfo::IsFitToRequirements(Unit const* clicker, Unit const* clicke
 ObjectMgr::ObjectMgr():
     _auctionId(1),
     _equipmentSetGuid(1),
-    _itemTextId(1),
     _mailId(1),
     _hiPetNumber(1),
     _creatureSpawnId(1),

src/server/game/Globals/ObjectMgr.h

@@ -1341,9 +1341,8 @@ class TC_GAME_API ObjectMgr
         // first free id for selected id type
         uint32 _auctionId;
         uint64 _equipmentSetGuid;
-        uint32 _itemTextId;
-        uint32 _mailId;
-        uint32 _hiPetNumber;
+        std::atomic<uint32> _mailId;
+        std::atomic<uint32> _hiPetNumber;
 
         uint32 _creatureSpawnId;
         uint32 _gameObjectSpawnId;