Update submodule sitl_gazebo to latest Sun Mar 21 12:39:01 UTC 2021

    - sitl_gazebo in PX4/Firmware (6e9f745): PX4/PX4-SITL_gazebo-classic@b195315
    - sitl_gazebo current upstream: PX4/PX4-SITL_gazebo-classic@1f339cd
    - Changes: PX4/PX4-SITL_gazebo-classic@b195315...1f339cd

    1f339cd 2021-03-20 Dongoo Lee - Pass ros_distro in CMakeLists.txt instaed of checking it in jinja_gen.py (#712)
1b1afca 2021-03-18 David Jablonski - gst camera: add RTMP streaming and nvidia encoding (#727)