docs/conf.py: update copyright date to 2022
docs/conf.py: update copyright date to 2022

While at it, mention other smaller contributions with the term 'the RAUC
contributors'.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

ejoerns/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc,rauc/rauc,rauc/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc