MAINTAINERS: Update wpan tree

MAINTAINERS: Update wpan tree

The wpan maintainers group is switching from Stefan's tree to a group
tree called 'wpan'. We will now maintain:
* wpan/wpan.git master:
  Fixes targeting the 'net' tree
* wpan/wpan-next.git master:
  Features targeting the 'net-next' tree
* wpan/wpan-next.git staging:
  Same as the wpan-next master branch, but we will push there first,
  expecting robots to parse the tree and report mistakes we would have
  not catch. This branch can be rebased and force pushed, unlike the
  others.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Alexander Aring <aahringo@redhat.com>
[Fixed two small typos stefan@datenfreihafen.org]
Link: https://lore.kernel.org/r/20230411090122.419761-1-miquel.raynal@bootlin.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>