fix: registeredDomain failing on repeated contact_id updates

[alexhuk3]

What

Fixes an issue where updating contact_id on a dnsimple_registered_domain succeeds on the initial change,
but fails on subsequent updates with:

error: unexpected unknown property value for "registrantChange"

Why

When a domain’s registrant contact is changed for the first time, the update completes successfully and a
registrantChange object is persisted in state.

On subsequent contact_id updates, the provider may encounter an unknown value for the
registrantChange attribute during planning, which causes the update to fail.

dnsimple:index:RegisteredDomain ... updating [diff: ~contactId]
error: unexpected unknown property value for "registrantChange"

This happens because:

• The registrant_change object did not preserve its prior state when its value was unknown during planning
• The update logic attempted to converge an already completed registrant change instead of creating a new one
• Several nested attributes were marked with RequiresReplace() despite representing historical
  registrant change data rather than the domain itself

This prevents repeated registrant contact changes from being applied reliably.

How

schema.go

• Added objectplanmodifier.UseStateForUnknown() to the registrant_change attribute to preserve the state value during planning
• Removed RequiresReplace() from contact_id and domain_id nested attributes
• Removed RequiresReplaceIfConfigured() from extended_attributes

update.go

• Updated the convergence logic to only attempt convergence for pending registrant changes
• Always create a new registrant change when contact_id differs, even if a previous change exists in a completed state

[DXTimer]

Thank you for the contribution.

Would you mind adding a test case that covers the described scenario? This would help us guard against regressions going forward.