batman-adv: Start new development cycle

This version will contain all the (major or even only minor) changes for
Linux 5.15.

The version number isn't a semantic version number with major and minor
information. It is just encoding the year of the expected publishing as
Linux -rc1 and the number of published versions this year (starting at 0).

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>