Update min selectable number for subscription buy process

What does this MR do?

Closes customers-gitlab-com#2374 (closed)

Update a minimum selectable number for purchasing an ultimate subscription. Guest users should be deducted from overall members count in this case.

Screenshots or Screencasts (strongly suggested)

Group with guests

Ultimate (Min 3)	Non-Ultimate (Min 4)

How to setup and validate locally (strongly suggested)

1. Sign in and create a new group on http://127.0.0.1:3000/groups/new. Add at least one guest user to the group. It's important that signed in user is an owner of this group.
2. In CustomersDot, purchase a SaaS subscription for the group. For Ultimate plan the number of guests should be deducted from the minimum selectable ammount.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.