Core/AI: Some more refactoring prep for #19930. CreatureAI::EnterCombat is now called CreatureAI::JustEngagedWith. There's also two new methods on UnitAI, though they're never invoked right now.

(cherry picked from commit 6113b9dec2e0081c36c36c2974a28ed0693bc601)

src/server/game/AI/ScriptedAI/ScriptedCreature.cpp

@@ -480,7 +480,7 @@ void BossAI::_JustReachedHome()
     me->setActive(false);
 }
 
-void BossAI::_EnterCombat()
+void BossAI::_JustEngagedWith()
 {
     if (instance)
     {
@@ -593,7 +593,7 @@ void WorldBossAI::_JustDied()
     summons.DespawnAll();
 }
 
-void WorldBossAI::_EnterCombat()
+void WorldBossAI::_JustEngagedWith()
 {
     Unit* target = SelectTarget(SELECT_TARGET_RANDOM, 0, 0.0f, true);
     if (target)