Make telephone optional on in-app trial forms

What does this MR do and why?

Make telephone optional on in-app trial forms

• phone number should be optional, but still validate pattern.

• make errors pretty - move away from default html5 form validation from browser.

• aligned with the new user version of this form performed in Update the New User Registration Trial /company... (!188164 - merged)

• see https://gitlab.com/gitlab-org/gitlab/-/issues/527169

Changelog: other EE: true

Screenshots or screen recordings

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item	Before	After	Video
/-/trials/new			Screen_Recording_2025-05-12_at_2.39.54_PM
/-/trials/duo_enterprise/new			Screen_Recording_2025-05-12_at_2.41.22_PM
/-/trials/duo_pro/new			Screen_Recording_2025-05-12_at_2.42.41_PM
general form validation updates	Screen_Recording_2025-05-12_at_2.49.54_PM	Screen_Recording_2025-05-12_at_2.50.36_PM	n/a

How to set up and validate locally

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

Ultimate trial

1. visit /-/trials/new path.

Duo Enterprise trial

1. Create a new top-level group
2. In the gitlab admin UI, edit the group to be on an ultimate plan(edit button from http://gdk.test:3000/admin/groups)
3. visit /-/trials/duo_enterprise/new path.

Duo Pro trial

1. Create a new top-level group
2. In the gitlab admin UI, edit the group to be on a premium plan(edit button from http://gdk.test:3000/admin/groups)
3. visit /-/trials/duo_pro/new path.

MR acceptance checklist

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

Related to #527169