fix(Core/LFG): Hide XP from max players (#18103)

* Cherry-pick Player::IsMaxLevel from TrinityCore
* Correctly hide XP reward for max level players in LFG
* Closes #18101

src/server/game/Entities/Player/Player.h

@@ -1686,6 +1686,7 @@ public:
     void SetFreeTalentPoints(uint32 points);
     bool resetTalents(bool noResetCost = false);
     [[nodiscard]] uint32 resetTalentsCost() const;
+    bool IsMaxLevel() const;
     void InitTalentForLevel();
     void BuildPlayerTalentsInfoData(WorldPacket* data);
     void BuildPetTalentsInfoData(WorldPacket* data);