Use partial variable instead of instance variable

What does this MR do and why?

Use partial variable instead of instance variable

It might be better isolation here to pass in the magical @group as namespace here when using it in a partial as it can become hard to follow / understand how they are set when used outside of the parent template.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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Ultimate	Ultimate trial

How to set up and validate locally

• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
• Sign in. Create 1 group.

bin/rails c

Feature.enable(:duo_pro_trials)
Feature.enable(:cs_connect_with_sales)
GitlabSubscriptions::AddOn.find_or_create_by(name: 'code_suggestions', description: GitlabSubscriptions::AddOn.descriptions[:code_suggestions])
GitlabSubscription.last.update(hosted_plan: Plan.find_by(name: 'ultimate'))

• Visit created group billing page.
• Make sure Start a Duo Pro trial and Contact sales buttons work.

bin/rails c

GitlabSubscription.last.update(hosted_plan: Plan.find_by(name: 'ultimate_trial'), trial_starts_on: 1.days.ago, trial_ends_on: 29.days.from_now, trial: true)

• Reload page.
• Make sure Start a Duo Pro trial and Contact sales buttons work.

Related to #451267 (closed)