- use `gazebo-classic` everywhere consistently referring to the original Gazebo (eg version 9,10,11)
- additional `gazebo_*` helper targets added for compatibility, but warn about deprecation and tell you the new target naming
- use `gz` everywhere when referring to Gazebo (aka Ignition Gazebo or new Gazebo)
- for convenience merge px4_sitl_ign into px4_sitl_default, but allow simulator_ignition_bridge to quietly skip inclusion if ignition-transport isn't available
- simulator_ignition_bridge only try setting the system clock in
lockstep builds
- this simplifies usage and CI system dependencies
Daniel Agar