Fix VRF gold BGP IPv4 activation on cores and border leafs

With 'no bgp default ipv4-unicast' set at the router level, VRF gold
eBGP/iBGP neighbors were establishing but not exchanging any IPv4
prefixes, breaking inter-fabric transit between DC and Campus. Add an
explicit 'address-family ipv4' block with 'neighbor X activate' under
'vrf gold' on both cores and all four border leafs.

Also drop 'redistribute learned' from the border leaf VRF gold stanza:
it is not a valid command in that context and was silently stripped by
EOS.

configs/border-leaf-dc2.cfg

@@ -228,7 +228,11 @@ router bgp 65005
       neighbor 10.0.14.6 remote-as 65500
       neighbor 10.0.14.6 send-community extended
       neighbor 10.0.14.6 maximum-routes 12000 warning-only
+      !
+      address-family ipv4
+         neighbor 10.0.14.2 activate
+         neighbor 10.0.14.6 activate
+      !
       redistribute connected
-      redistribute learned
 !
 end