Move actual_shared_runners_minutes_limit to Ci::Minutes::Quota

What does this MR do?

Related to #213964 (closed)

We are in process of moving a lot of the logic about CI minutes that was implemented in Namespace, NamespaceStatistics and NamespaceHelpers into a single class Ci::Minutes::Quota. This refactoring will help us making changes into how minutes are calculated, reset, enforced and manged.

In this MR we move Namespace#actual_shared_runners_minutes_limit to Ci::Minutes::Quota#total_minutes.

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

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• 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
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