MAINTAINERS: mark ISDN subsystem as orphan

MAINTAINERS: mark ISDN subsystem as orphan

We have not heard any activities from Karsten in years:

  - Last review tag was nine years ago in commit a921e9bd4e22a7
    ("isdn: i4l: move active-isdn drivers to staging")
  - Last message on lore was in October 2020 [1].

Furthermore, messages to isdn mailing list bounce.

Mark the subsystem as orphan to reflect these.

[1]: https://lore.kernel.org/all/0ee243a9-9937-ad26-0684-44b18e772662@linux-pingi.de/

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20251023092406.56699-1-bagasdotme@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>