Fix namespace checks for live quota consumption

What does this MR do?

Related to #339020 (closed) and gitlab-com/gl-infra/production#5396 (closed)

The check Namespace#unlimited_minutes? defined to fix a production incident was not representing the SSOT. Ci::Minutes::Quota#enabled? instead is the SSOT because it considers whether the CI minutes limit is not explicitly set at the namespace level (being NULL) hence defaulting to the Gitlab::CurrentSetting.shared_runners_minutes.

This caused another bug where on gitlab-staging we had namespaces with NULL namespace-level minutes and the condition unlimited_minutes? returned false while it should have been true.

In this MR we fix the bug by moving the check of Ci::Minutes::Quota#enabled? into Ci::Minutes::CostFactor class as per !68599 (comment 655862191). In other words, if a namespace has unlimited minutes, any jobs running in it should have a cost factor of 0, meaning we don't track CI minutes consumption as well as we skip enforcing CI minutes quota.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

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?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• 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.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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