Fix group name bug for new purchase flow

What does this MR do?

Some users are getting Failed to confirm your order! Please try again. while attempting the new purchase flow.

The new registration with checkout flow will create a group name automatically here

The created group name will not always complain with the group_name_regex. The save fails and we get a cryptic error message

What the proposed fix is

Build a group name that complains with the group_name_regex

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Contributes #234091 (closed)