Inconsistent terminology for Heroku App ID

[chrylis]

The current code uses SPRING_CLOUD_APP_NAME to identify an application name from within Heroku, but the API refers to the value in question as appId(). Keep as a wart or breaking-change to a consistent SPRING_CLOUD_APP_ID?

Note that the way I determine whether the local connector should activate is to look for spring.cloud.appId; if we extend that to an environment scan, the values would semantically mean the same thing, but the current logic would get a false-positive on the local connector, so that would need to be addressed somehow.

[ramnivas]

Heroku connector isn't as widely used so far, so making this change (deprecate SPRING_CLOUD_APP_NAME with a runtime warning) in favor of SPRING_CLOUD_APP_ID is fine. If you can make a PR, that will be great. Else, I will handle this shortly.

[scottfrederick]

Since this library has been released for a while now and is in wider use, let's not implement this breaking change.