Save namespace_storage_forks_cost_factor Setting in Admin Controller

What does this MR do and why?

PUT requests to the EE::Admin::ApplicationSettingsController#update method that include a value for the namespace_storage_forks_cost_factor setting will save the value after this MR.

This will allow the the admin UI in https://gitlab.com/gitlab-org/gitlab/-/issues/404628 to save the value.

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

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.