CODEOWNERS: Fix shields ownership overwrite

This commit fixes an issue with shield ownership assignments in
the CODEOWNERS file:

- Moved the general shields pattern (**/shields/) above specific
  shield entries
- Modified specific shield entries to include both teams:
  - Added @nrfconnect/ncs-co-build-system to each specific shield entry
  - Preserved original team assignments (@nrfconnect/ncs-low-level-test
     and @nrfconnect/ncs-radio-sw)
- Removed misleading comment about shield entry positioning

The previous configuration caused specific shield owners to be
overwritten by the general @nrfconnect/ncs-co-build-system entry.

Signed-off-by: Dawid Przybylo <dawid.przybylo@nordicsemi.no>

Author: dawidprzybylo

CODEOWNERS

@@ -54,14 +54,14 @@
 /boards/nordic/nrf54l*/                   @nrfconnect/ncs-co-boards @kl-cruz
 /boards/nordic/nrf52*                     @nrfconnect/ncs-co-boards @nrfconnect/ncs-si-bluebagel
 /boards/nordic/thingy91*/                 @nrfconnect/ncs-co-boards @nrfconnect/ncs-cia
-/boards/shields/coverage_support/         @nrfconnect/ncs-low-level-test
-/boards/shields/nrf2220ek/                @nrfconnect/ncs-radio-sw
-/boards/shields/nrf2240ek/                @nrfconnect/ncs-radio-sw
-/boards/shields/nrf21540ek/               @nrfconnect/ncs-radio-sw
 
-# This shield entry must be below other shield entries (excluding doc), do not move
 **/shields/                               @nrfconnect/ncs-co-build-system
 
+/boards/shields/coverage_support/         @nrfconnect/ncs-co-build-system @nrfconnect/ncs-low-level-test
+/boards/shields/nrf2220ek/                @nrfconnect/ncs-co-build-system @nrfconnect/ncs-radio-sw
+/boards/shields/nrf2240ek/                @nrfconnect/ncs-co-build-system @nrfconnect/ncs-radio-sw
+/boards/shields/nrf21540ek/               @nrfconnect/ncs-co-build-system @nrfconnect/ncs-radio-sw
+
 /boards/shields/pca63566/doc/*.rst        @nrfconnect/ncs-doc-leads
 
 # SUIT configuration files