Core/Instances: Fix build in older VS versions

Relax const uses in BossBoundaryEntry so the compiler can generate proper
copy assignment operator (error C2582: 'operator =' function is unavailable in 'BossBoundaryEntry')

Also renamed its members to match our coding standards
(public members should be UpperCamelCase)

src/server/game/Instances/InstanceScript.cpp

@@ -105,8 +105,8 @@ void InstanceScript::SetHeaders(std::string const& dataHeaders)
 void InstanceScript::LoadBossBoundaries(const BossBoundaryData& data)
 {
     for (BossBoundaryEntry const& entry : data)
-        if (entry.bossId < bosses.size())
-            bosses[entry.bossId].boundary.insert(entry.boundary);
+        if (entry.BossId < bosses.size())
+            bosses[entry.bossId].boundary.insert(entry.Boundary);
 }
 
 void InstanceScript::LoadMinionData(const MinionData* data)