Revert "package/network-manager: bump to version 1.21.2"

The "1.21.2" is not a NetworkManager release, it's a development
snapshot. Bring the package back to 1.20.x stable series.

This reverts commit 9cf61746.
It should eventually be brought back, but just not yet.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>