Extract CI::Minutes::Limit class

What does this MR do and why?

Related to #277447 (closed)

This MR extracts a class that deals exclusively with CI minutes limits.

While working on recalculate purchased CI minutes lazily I realized that there could be a concerning loop between:

1. Ci::Minutes::Quota#current_usage calls Ci::Minutes::NamespaceMonthlyUsage.find_or_create_current
2. Ci::Minutes::NamespaceMonthlyUsage.find_or_create_current needs to recalculate remaining purchased CI minutes
3. The recalculation of purchased CI minutes invoked Ci::Minutes::Quota class (not current_usage)

While this doesn't represent a bug, since current_usage is not used when recalculating purchased CI minutes, it could be a problem if we make future changes that require current_usage.

To avoid this problem I've extracted Ci::Minutes::Limit class so it can be used instead of Ci::Minutes::Quota when recalculating purchased CI minutes: Quota#current_usage -> NamepaceMonthlyUsage.find_or_create_current -> Limit.recalculate_purchased_minutes!

The recalculate_purchased_minutes! will be introduced in the follow-up MR !73338 (merged).

In a follow-up issue we will need to rename Ci::Minutes::Quota with Ci::Minutes::Usage since "quota" and "limit" are synonyms. Our current Quota class deals with anything related to reading CI minutes data.

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