Allow some extra users when registering a new license

What does this MR do?

Context

Currently some customers are having issues when they try to register their license key after the paperwork has been finished between the company and GitLab for example. This happens because in that lapse of time the customer has been adding users and when they try to register the license key the total of users exceeds the number allowed by the license key. With this change, customers will be able to add up to 10% more of users on top of the limit restricted by the license key without receiving a validation error when registering the license key.

Changes

The change basically updates how the restricted users count is calculated by taking into into account the 10% of extra users allowed.

REF: https://gitlab.com/gitlab-org/license-gitlab-com/-/issues/184

Screenshots (strongly suggested)

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

Related to #184 (closed)