Refactor project shared_runners_seconds to use new tracking

What does this MR do and why?

Related to #277448 (closed)

Feature flag issue: #341730 (closed)

This MR is a preparation refactoring to eventually replace the usage of shared_runners_seconds with the new monthly tracking that uses Ci::Minutes::ProjectMonthlyUsage and Ci::Minutes::NamespaceMonthlyUsage.

In this MR we are reducing the reads of shared_runners_seconds at project level to a single method Project#ci_minutes_used that is guarded by feature flag ci_use_new_monthly_minutes.

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