Simplify CODEOWNERS (#888)

Simplify the list of entries for CODEOWNERS and add lumag + mwasilew.

Author: ricardosalveti

.github/CODEOWNERS

@@ -1,12 +1,6 @@
 # Default reviewers for all changes in entire repository,
 # unless a later match takes precedence
-*  @vkraleti @ricardosalveti @sbanerjee-quic @ndechesne
+*  @vkraleti @ricardosalveti @sbanerjee-quic @ndechesne @lumag
 
 # Reviewers to be assigned for any changes in ci/ subdirectory
-ci/*  @quaresmajose @ricardosalveti @sbanerjee-quic @ndechesne
-
-# Reviewers to be assigned for any changes in conf/ subdirectory
-conf/*  @vkraleti @ricardosalveti @sbanerjee-quic @ndechesne
-
-# Reviewers to be assigned for any changes in recipes-kernel/linux/ subdirectory
-recipes-kernel/linux/*  @NainaMehtaQUIC @vkraleti @sbanerjee-quic @ndechesne
+ci/*  @quaresmajose @ricardosalveti @sbanerjee-quic @ndechesne @mwasilew @lumag