schema update: manage internal/external DNS

[davepacheco]

Not exactly part of schema update, but related to the same handoff.

• We should not include new-version Nexus instances in internal or external DNS before the handoff
• We should not include old-version Nexus instances in internal or external DNS after the handoff

[karencfv]

Might need #8863 for this

[karencfv]

Will unassign myself from this for now