Multi-interface servers blueprint updates

[ocket 8888 <oc...@gmail.com>]

The blueprint needs to be updated:
https://github.com/apache/trafficcontrol/pull/4688

The changes contain an important notice to CDN operators; upgrading to a
version of ATC that includes this feature (expected to be 5.0) will require
a check for invalid data prior to upgrading - or the upgrade WILL fail. For
specifics, look into the blueprint.

Also, management IPs are no longer being converted automatically into the
new "interfaces" array, as they are currently unvalidated in
TP/TO-API/TO-DB and cannot be guaranteed to have valid values. So instead
of removing those fields, they are to be marked as deprecated and operators
will need to perform a manual conversion of all invalid values into the new
interfaces array (valid values can still be done automatically by a
migration when the time comes - so just making sure everything is valid is
also a solution).