Use interactions in community programs endpoints

Currently, we are using service as the main tool to run the logic behind a community program requests. Instead of using services directly we should call an interaction first for validation layer. See example here.

This issue covers the following files:

1. app/controllers/subscriptions/community_program/coupons_controller.rb
2. app/controllers/subscriptions/community_program/namespaces_controller.rb

More information in the original thread here: https://gitlab.com/gitlab-com/marketing/developer-relations/strategy-programs/open-source-program/gitlab-for-open-source/-/issues/51#note_2473519230