Consider single source of truth for checking valkey backend

The following discussion from !9113 (merged) should be addressed:

• @Alexand started a discussion:

  non-blocking follow-up

  Like with the last MR, we're seeing node['redis']['backend'] == 'valkey' being defined in three different places:

  • files/gitlab-cookbooks/gitlab-ee/libraries/sentinel_helper.rb
  • files/gitlab-cookbooks/package/libraries/helpers/redis_helper/server.rb
  • files/gitlab-cookbooks/redis/templates/default/sv-redis-run.erb

  If I'm not mistaken, the package cookbook is supposed to be the shared package that gets loaded early for any other cookbook. So I believe we can track shared config logic there. But I don't think we should halt this MR due to its urgency, and now it's a much smaller change than before. But let's consider in a follow-up if we can create a SSoT for this logic that could be shared across cookbooks.