Update submodule mavlink to latest Wed Mar 23 12:59:24 UTC 2022

    - mavlink in PX4/Firmware (2193bfc): mavlink/mavlink@b568a60
    - mavlink current upstream: mavlink/mavlink@3d9594e
    - Changes: mavlink/mavlink@b568a60...3d9594e

    3d9594ef 2022-03-23 muexxl - add enums for mavlink ftp service (#1801)
d7a20cc3 2022-03-17 Hamish Willee - VTOL - define MAV_TYPE for belly-sitting non tiltrotor (#1808)
1fd97fb9 2022-03-17 Julian Oes - common: remove option for relative speed change (#1812)
8197285e 2022-03-16 Amilcar Lucas - Improve example documentation (#1816)
485f7d05 2022-03-14 Hamish Willee - Param protocol bits: revert float, move c_style (#1814)
1942c71e 2022-03-03 Hamish Willee - Tell git to ignore the doc/messages folder (#1809)
c38f6660 2022-03-03 Hamish Willee - Rename and document parameter encoding protocol bits. (#1799)
5186324e 2022-03-03 Julian Oes - camera/component information: URI zero termination (#1807)