Prevent manual upload of cloud license

What does this MR do?

Related to #325594

This MR prevents cloud licenses from being manually uploaded or entered in the admin panel.

• Updates the LicenseController#create action to prevent cloud licenses from being created.
• Presents an alert to the user to upload a valid license if a cloud license is used.
• Improve LicenseController specs with expectations on the creation of License records.

Screenshots (strongly suggested)

The following is the alert shown when a cloud license is used:

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because cloud licenses are created behind a feature flag.
• [-] 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