MAINTAINERS: Make nordicjm maintainer for MCUmgr subsystem

With significant contribution to MCUmgr that brought increased
upload speeds and various other improvements in RAM and CPU usage,
general improvements in MCUmgr architecture and documentation,
Jamie has shown his vision for the subsystem and therefore
lets welcome him as a new MCUmgr subsystem maintainer.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

CODEOWNERS

@@ -651,7 +651,7 @@
 /include/zephyr/logging/                         @nordic-krch
 /include/zephyr/lorawan/lorawan.h                @Mani-Sadhasivam
 /include/zephyr/mgmt/osdp.h                      @sidcha
-/include/zephyr/mgmt/mcumgr/                     @de-nordic
+/include/zephyr/mgmt/mcumgr/                     @nordicjm
 /include/zephyr/net/                             @rlubos @tbursztyka
 /include/zephyr/net/buf.h                        @jhedberg @tbursztyka @rlubos
 /include/zephyr/net/coap*.h                      @rlubos

MAINTAINERS.yml

@@ -1699,9 +1699,9 @@ Mbed TLS:
 MCU Manager:
   status: maintained
   maintainers:
-    - de-nordic
-  collaborators:
     - nordicjm
+  collaborators:
+    - de-nordic
   files:
     - subsys/mgmt/mcumgr/
     - include/zephyr/mgmt/mcumgr/