Follow-up from "Add name and description fields to member_roles"

The following discussions from !126423 (merged) should be addressed:

• @kassio started a discussion: (+3 comments)

  suggestion What do you think to make it more clear where these values come from, and as a side-effect reduce this line.

  We could use the existing Gitlab::Access.options_with_owner to tie together MemberRole and it's access levels, like:

  # ee/app/models/members/member_role.rb
  
  class MemberRole < ApplicationRecord
    # ...
  
    class << self
      # ...
  
      def levels
        ::Gitlab::Access
          .options_with_owner
          .values
      end
  
      def levels_sentence
        ::Gitlab::Access
          .options_with_owner
          .map { |name, value| "#{value} (#{name})" }
          .to_sentence
      end
    end
  
    # ...
  end

  [dev](main)> MemberRole.levels
  => [10, 20, 30, 40, 50]
  [dev](main)> MemberRole.levels_sentence
  => "10 (Guest), 20 (Reporter), 30 (Developer), 40 (Maintainer), and 50 (Owner)"

  And, with that, we can use the levels_sentence here:

            description: "Access level. Valid values are #{::MemberRole.levels_sentence}.",
            values: ::MemberRole.levels

  sidenote: I started just trying to reduce this line, but I end up with all that. 😬