Add helper to telephone number field in hand raise modal

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR clarifies why the phone number field is required to submit the contact sales form.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Use the rail console in GDK

   cd ../gitlab && ./bin/rails console

2. Make sure the billing page is accessible

   ApplicationSetting.first.update(check_namespace_plan: true)

3. Enable the contact sales modal

   Feature.enable(:in_app_hand_raise_pql)

4. Navigate to a group with owner permission

5. Click on Contact sales on any tier card

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 #334211 (closed)