Update conditions for disabling user numbers input

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR addresses the bug described in gitlab-org/gitlab#346969 by adding a condition for disabling the number of users field during the Subscription purchase flow. Read more details in the Issue. Note that specs have not changed because they were already setting the conditions right, although not actually testing them properly. It happens 😊

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

Before	After

Local setup is quite hard, unfortunately. I have tried several times to run the <code data-sourcepos="27:83-28:0">The only time that "just me" matters to the purchase process is during the paid signup flow </code> case but gdk gets quite stuck.

How to set up and validate locally

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

Local setup is quite hard, unfortunately. I have tried several times to run the <code data-sourcepos="35:83-36:0">The only time that "just me" matters to the purchase process is during the paid signup flow </code> case but gdk gets quite stuck.

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.