Deprecate ApplicationController#currentPath and ApplicationController#currentRouteName

[locks]

Tracks https://github.com/emberjs/rfcs/master/text/0421-deprecate-application-controller-props.md.

Tasks

• Implement deprecation in source code [DEPRECATE] - RFC 421 - deprecate app ctrl router props #17573
• Write deprecation guide Application controller router properties deprecation ember-learn/deprecation-app#276

[rwjblue]

We should migrate this to the corresponding emberjs/rfc-tracking issue now that they are created...

[locks]

See emberjs/rfc-tracking#5.