* Replace protobuf 2.6.1 with 3.21.8
* Update/add protobuf libs
* Add CS2 protos
* Remove old csgo/dota protos
* Add versioned protoc bin
* Comment out Valve's `schema` define for now
* Use ENetworkDisconnectionReason
* Fix-up `offsetof` to avoid errors on some Clang versions
Add CConcreteEntityList, CEntityComponent, CScriptComponent, CGameEntitySystem, rewrite IHandleEntity to use CEntityHandle instead of CBaseHandle, update NUM_SERIAL_NUM_BITS, comment out old CBaseEntity, obsolete basehandle.h