Update setup instructions to W21 template (`temp-credentials.txt`, SETUP-FULL, SETUP-QUICKSTART)

[pconrad]

Update the setup instructions to match the W21 template pioneered in the W21 STARTER-jpa03 repo and the demo-spring-react-minimal apps.

This change is mostly documentation, but there are two changes to configuration files (pom.xml and setHerokuEnv.py).

Acceptance Criteria:

• README.md points to SETUP-FULL, SETUP-QUICKSTART
• SETUP-FULL, SETUP-QUICKSTART are updated with instructions that go through all of the configuration first, putting values into temp-credentials.txt, then copying them into secrets-localhost.properties, javascript/.env.local and secrets-heroku.properties.
• Example app name(s) in the instructions are sensible for the context
• setHerokuEnv.py is updated so that javascript/.env.production is optional and the error messages are helpful
• pom.xml is updated so that the Heroku config var is called SPRING_PROPERTIES instead of HEROKU_PROPERTIES (the HEROKU_* namespace is reserved by Heroku