Add Mayra Cabrera to codeowners

Per the second step of:
https://gitlab.com/gitlab-com/www-gitlab-com/issues/3351

.gitlab/CODEOWNERS.disabled

 # Backend Maintainers are the default for all ruby files
-*.rb @ashmckenzie @ayufan @dbalexandre @DouweM @dzaporozhets @godfat @grzesiek @mkozono @nick.thomas @rspeicher @rymai @smcgivern
-*.rake @ashmckenzie @ayufan @dbalexandre @DouweM @dzaporozhets @godfat @grzesiek @mkozono @nick.thomas @rspeicher @rymai @smcgivern
+*.rb @ashmckenzie @ayufan @dbalexandre @DouweM @dzaporozhets @godfat @grzesiek @mkozono @nick.thomas @rspeicher @rymai @smcgivern @mayra-cabrera
+*.rake @ashmckenzie @ayufan @dbalexandre @DouweM @dzaporozhets @godfat @grzesiek @mkozono @nick.thomas @rspeicher @rymai @smcgivern @mayra-cabrera
 
 # Technical writing team are the default reviewers for everything in `doc/`
 /doc/ @axil @marcia