Add group creation to one step ultimate trial self-serve process

What does this MR do and why?

• add selector option and functionality for creating a group. This is the final feature for this issue.
• see https://gitlab.com/gitlab-org/gitlab/-/issues/532644

Screenshots or screen recordings

Click to expand

item	Before	After
core func
after selection	n/a
when selected list	n/a
behavior video	n/a	Screen_Recording_2025-06-12_at_9.28.37_AM

How to set up and validate locally

covered by feature specs...

1. Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
2. Enable ultimate_trial_single_form feature flag in rails console(bundle exec rails c).

   Feature.enable(:ultimate_trial_single_form)

3. To enable groups for selector, since we aren't connected to CustomersDot locally, change this line to be user.owned_groups.
4. visit /-/trials/new path.

MR acceptance checklist

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Related to #532644