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 a921e9bd4e
    ("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>

CREDITS

@@ -2036,6 +2036,10 @@ S: Botanicka' 68a
 S: 602 00 Brno
 S: Czech Republic
 
+N: Karsten Keil
+E: isdn@linux-pingi.de
+D: ISDN subsystem maintainer
+
 N: Jakob Kemi
 E: jakob.kemi@telia.com
 D: V4L W9966 Webcam driver

MAINTAINERS

@@ -13247,10 +13247,8 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/nab/target-pending.git mast
 F:	drivers/infiniband/ulp/isert
 
 ISDN/CMTP OVER BLUETOOTH
-M:	Karsten Keil <isdn@linux-pingi.de>
-L:	isdn4linux@listserv.isdn4linux.de (subscribers-only)
 L:	netdev@vger.kernel.org
-S:	Odd Fixes
+S:	Orphan
 W:	http://www.isdn4linux.de
 F:	Documentation/isdn/
 F:	drivers/isdn/capi/
@@ -13259,10 +13257,8 @@ F:	include/uapi/linux/isdn/
 F:	net/bluetooth/cmtp/
 
 ISDN/mISDN SUBSYSTEM
-M:	Karsten Keil <isdn@linux-pingi.de>
-L:	isdn4linux@listserv.isdn4linux.de (subscribers-only)
 L:	netdev@vger.kernel.org
-S:	Maintained
+S:	Orphan
 W:	http://www.isdn4linux.de
 F:	drivers/isdn/Kconfig
 F:	drivers/isdn/Makefile