Update condition for submit button on 'just me' cases

What does this MR do and why?

Describe in detail what your merge request does and why.

Relates to #348159 (closed). After this MR (!76112 (merged)) the Submit button is not disabling unless the number of users is 1. Due to the possibility that just me group add more users, this may cause disruption in cases of Plan purchase.

With this MR we're also adding more coverage for number of users cases and updating the readability of conditions.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Sign up for a new gitlab.com account
2. During the first step of onboarding, select Just me as the answer to Who will be using GitLab?
3. Complete signup by naming your new project and group
4. Navigate to the group that you just created
5. Navigate to the Members page of the group you just created and add a member as a developer role. You should now have 2 members.
6. Navigate to the Billing page of the group and select Upgrade for either Premium or Ultimate.
7. Observe that the Continue to billing button is enabled but only in case it's bigger than amount of users in the group.

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.