Adjust show_code_suggestions_tab? to use code_suggestions_available?

What does this MR do and why?

In this MR we adjust show_code_suggestions_tab? to:

• Use code_suggestions_available?
• Remove Gitlab.com? and use Sass feature check instead
• Adjust the specs

How to set up and validate locally

Go to any group page, enable/disable the feature flags, and check the code_sugegstions tab on the usage quota page for SaaS.

On SaaS, pick a group:

• gdk stop rails-web && GITLAB_RAILS_RACK_TIMEOUT_ENABLE_LOGGING=false PUMA_SINGLE_MODE=true GITLAB_SIMULATE_SAAS=1 gdk rails s
• Feature.enable(:hamilton_seat_management) # Check example http://gdk.test:3000/groups/gitlab-instance-3513b16a/-/usage_quotas#seats-quota-tab
• Feature.disable(:hamilton_seat_management) # Check example http://gdk.test:3000/groups/gitlab-instance-3513b16a/-/usage_quotas#seats-quota-tab

On SM, pick a group: verify that the code suggestions tab is not showing.

• gdk stop rails-web && GITLAB_RAILS_RACK_TIMEOUT_ENABLE_LOGGING=false PUMA_SINGLE_MODE=true GITLAB_SIMULATE_SAAS=0 gdk rails s
• Feature.enable(:self_managed_code_suggestions) # Check example On a [not paid plan](https://gdk.test:3000/groups/gitlab-instance-3513b16a/-/usage_quotas#seats-quota-tab

Note: Wait a couple of seconds after each enable/disable.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #433011