Add remove_dormant_members settings to namespace_settings

What does this MR do and why?

This implements [BE] Add dormant member setting to Groups (#461334 - closed)

We're adding these settings to be used in #461339 (closed) and it will control the new feature we're adding in &13668

MR Cadence

Description	MR
Add remove_dormant_members_period setting to namespace_settings	👈 you're here
Allow Groups to update remove_dormant_members_period	!153118 (merged)

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