Verify group name is valid inside of the purchase flow

What does this MR do and why?

Group Owners have the ability to create a new group inside of the purchase flow when they are selecting which group they want to apply their GitLab subscription tier. Currently we do not apply any sort of validation to ensure the group name is usable. Apply validation to group name when a new group is created inside of the checkout flow.

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Login and go to /-/subscriptions/new
2. Select new group. Fill it in with ###. Fill in the rest of the form. Click Confirm purchase
3. See the error.

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 #346962 (closed)