Change trial group selection placeholder text

What does this MR do and why?

Change trial group selection placeholder text

• as per TW request change the text to Select a group
• this affects the following pages:
  • /-/trials/duo_enterprise/new?step=trial
  • /-/trials/duo_pro/new?step=trial
  • /-/trials/new?step=trial
• see #480093 (closed)

Changelog: other EE: true

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Before	After

How to set up and validate locally

Note: the steps below are for duo enterprise only, but should be good enough to validate overall as the feature specs cover all cases.

1. Setup GDK to simulate SaaS.

2. Enable duo_enterprise_trials feature flag in bin/rails console with Feature.enable(:duo_enterprise_trials).

3. Seed the add-ons for your instance from the terminal if they haven't been created yet(rerun will not harm anything if you do have them)

   • FILTER=add_ons bundle exec rake db:seed_fu

4. Create new group.

5. In rails console make it ultimate.

bin/rails c

Group.last.gitlab_subscription.update(hosted_plan: Plan.find_by(name: 'ultimate'))

6. Create another new group and repeat step #5 above.
7. Visit /-/trials/duo_enterprise/new?step=trial. New page.
8. See form.

Related to #480093 (closed)