Show a better message when a subgraph is published with no changes

[bonnici]

If a rover subgraph publish is run but there are no changes to the schema or routing URL, the publish mutation will succeed but nothing is actually changed related to the subgraph and a build/launch is never created. We indicate that this is the case by sending back wasUpdated: false but rover is not currently using that field (and it's not even exposed via the platform-api). This has caused some confusion because when customer's see the "the subgraph has was updated" message they think that something must have gone wrong during composition. This can be reproduced by simply running a subgraph publish twice for the same schema and URL:

> APOLLO_KEY=x \
  rover subgraph publish x@x \
  --schema ./schema.graphql \
  --name subgraph-name \
  --routing-url http://some.url.here.com
Publishing SDL to x@x (subgraph: subgraph-name) using credentials from the default profile.
The 'subgraph-name' subgraph in 'x@x' was updated
The supergraph schema for 'x@x' was NOT updated with a new schema
> APOLLO_KEY=x \
  rover subgraph publish x@x \
  rover subgraph publish x@x \
  --schema ./schema.graphql \
  --name subgraph-name \
  --routing-url http://some.url.here.com
Publishing SDL to x@x (subgraph: subgraph-name) using credentials from the default profile.
The 'subgraph-name' subgraph in 'x@x' was updated
The supergraph schema for 'x@x' was NOT updated with a new schema

This PR updates the message shown after the second publish above so that it would instead say:

Publishing SDL to x@x (subgraph: subgraph-name) using credentials from the default profile.
The 'subgraph-name' subgraph was NOT updated because no changes were detected
The supergraph schema for 'x@x' was NOT updated with a new schema

This was tested by using:

APOLLO_REGISTRY_URL=x \
  APOLLO_KEY=x \
  cargo run subgraph publish x@x \
  --schema ~/some/path/booking-schema.graphql \
  --name booking-subgraph \
  --routing-url http://some.url.here.com

On my staging graph and repeating it with no changes, then repeating it again after making a change to the schema.