Update the supported version to 21.12, matching the version used in
stx.6.0.
Testing:
Full build
Upgrade from stx.6.0
AIO-SX bootstrap + deploy
Story: 2009754
Task: 44223
Signed-off-by: David Sullivan <david.sullivan@windriver.com>
Change-Id: Ie001cf3373d5c14189c3976488d5196f6e3f86eb