update CODEOWNERS as per STABLE_RELEASES.md

Launchpad's Stable Release Managers team's current composition as
approved by @okwme (ICF):

* @alessio
* @clevinson
* @ethanfrey

.github/CODEOWNERS

@@ -1,7 +1,7 @@
 # CODEOWNERS: https://help.github.com/articles/about-codeowners/
 
 # Primary repo maintainers
-*       @rigelrozanski @alexanderbez @jackzampolin @alessio @fedekunze
+*       @alessio @clevinson @ethanfrey
 
 ###############################################################################
 # Module Specific Ownership