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.cpp

@@ -2524,6 +2524,11 @@ void Player::GiveLevel(uint8 level)
     sScriptMgr->OnPlayerLevelChanged(this, oldLevel);
 }
 
+bool Player::IsMaxLevel() const
+{
+    return GetLevel() >= GetUInt32Value(PLAYER_FIELD_MAX_LEVEL);
+}
+
 void Player::InitTalentForLevel()
 {
     uint32 talentPointsForLevel = CalculateTalentsPoints();