Add Validation for App Setting namespace_storage_forks_cost_factor

What does this MR do and why?

Add validation for namespace_storage_forks_cost_factor application setting.

Ensure value is between 0 and 1.

This application setting was added to the application_settings table in !126259 (merged).

It is not used for anything yet. A constraint added in !126259 (merged) keeps the value between 0 and 1 at the database layer. This MR adds a rails validation for good error handling at the application layer; otherwise we just hit the constraint at the database and throw an error.

The default value is 1.0.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/404628

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