Allows new purchase flow to handle upgrades

What does this MR do?

Allows new purchase flow to handle upgrades !29232 (comment 328773832)

• We should allow an existing user to initiate and return back to a proper screen with a custom redirect
• Also @kcomoli comment there !29232 (comment 328979637) we should make sure we get the correct flow at the end of the payment.
• We should also make sure personal & group namespaces are handled properly if needed

Suggestion

suggestion (non-blocking): we can simplify this code, if we pass groups default value as []. (this will work if groups is undefined or not provided, but will not work if groups is null) This will allow us to eliminate extra early return See !29232 (comment 337222479)

Screenshots

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

Closes #211423 (closed)