Update child objects when updating web_based_commit_signing_enabled

What does this MR do and why?

In a previous MR we introduced a job to update the web_based_commit_signing_enabled setting for child objects of the group. This change is to enqueue that job when the setting changes on the group.

References

• https://docs.gitlab.com/development/cascading_settings/#recommendation-for-cascading-settings-writes-going-forward

MR acceptance checklist

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Related to #540912 (closed)