[Consistency] Update code samples to use `APP_NAME` instead of `myapp`

[gilzow]

Where on docs.platform.sh should be changed?

everywhere, both platform and upsun

What exactly should be updated?

some of our code samples use APP_NAME for the app name to designate the name: value is variable. However, other areas now use myapp as the name: in code samples (and there are a few sections that use both). The inconsistency has the potential to cause confusion among new users. All code samples should stick to the convention of using APP_NAME.

Additional context

https://www.google.com/search?q=myapp+site%3Adocs.upsun.com&oq=myapp+site%3Adocs.upsun.com&gs_lcrp=EgZjaHJvbWUyBggAEEUYOdIBCDYzODRqMGo0qAIAsAIB&sourceid=chrome&ie=UTF-8