Fix calculation of used extra CI minutes

What does this MR do?

When a Group/Personal namespace is using the Free plan on GL.com then the assigned CI minutes is read from Gitlab::CurrentSettings.shared_runners_minutes and not from Namespace#shared_runners_minutes.

In the previous version we were using Namespace#shared_runners_minutes_limit which was returning nil instead of 2000 (The current default CI minutes configured globally for free plans in GL.com). This bug was affecting Pipelines and was showing wrong information on the Pipeline Quote info for the Group/Personal namespace.

Now we're using Namespace#actual_shared_runners_minutes_limit which returns the globally configured value if the current Namespace has not been configured with CI minutes.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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

REF: gitlab-org/customers-gitlab-com#494