Move web_based_commit_signing_enabled behind new feature flag

What does this MR do and why?

The use_web_based_commit_signing_enabled feature flag controlled both the ability to configure the setting for web based commit signing as well as the behavior that would is controlled by that setting.

This change introduces the configure_web_based_commit_signing feature flag to control those things separately

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #542975