Update submodule mavlink to latest Thu Mar 31 00:39:03 UTC 2022

    - mavlink in PX4/Firmware (b4e7226): mavlink/mavlink@b568a60
    - mavlink current upstream: mavlink/mavlink@20626df
    - Changes: mavlink/mavlink@b568a60...20626df

    20626dfa 2022-03-24 Hamish Willee - AVSSUAS.xml - remove empty whitespace at end of enum names
d5b9709e 2022-03-24 Peter Barker - common.xml: remove wip from AIS_VESSEL (#1817)
ee1bf9de 2022-03-24 Hamish Willee - MESSAGE_INTERVAL can be got using MAV_CMD_GET_MESSAGE_INTERVAL or MAV_CMD_REQUEST_MESSAGE (#1822)
087c1167 2022-03-24 Peter Barker - AVSUAS: correct MODE_M300_FORCE_AUTO_LANDING enum entry (#1824)
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)