[google-c2p resolver] ignore xDS resource deletion (#32984)

This enables the feature described in [gRFC
A53](https://github.com/grpc/proposal/blob/master/A53-xds-ignore-resource-deletion.md)
for C2P.

This also removes the now-unnecessary `xds_v3` server feature, which
should have been removed as part of #31327.

src/core/ext/filters/client_channel/resolver/google_c2p/google_c2p_resolver.cc

@@ -247,7 +247,8 @@ void GoogleCloud2ProdResolver::StartXdsResolver() {
                    {"type", Json::FromString("google_default")},
                }),
            })},
-          {"server_features", Json::FromArray({Json::FromString("xds_v3")})},
+          {"server_features",
+           Json::FromArray({Json::FromString("ignore_resource_deletion")})},
       }),
   });
   Json bootstrap = Json::FromObject({