Heroku error running Play: controllers/routes has been compiled by a more recent version of the Java Runtime - scala

Question

I am trying to run Play on Heroku...
The full error is:
play.api.UnexpectedException: Unexpected exception[RuntimeException: java.lang.UnsupportedClassVersionError: controllers/routes has been compiled by a more recent version of the Java Runtime (class file version 58.0), this version of the Java Runtime only recognizes class file versions up to 55.0] (58 is Java 14 and 55 is Java 11).
The Heroku help says:
Make sure you are using the correct JDK version by setting it in the system.properties file.
I added the following to system.proprties:
java.runtime.version=14.0.1.
And I can see it gets used by the Heroku compile. But I still get the error.
I thought that maybe the problem was the other way around so I also tried:
java.runtime.version=11.0.8
I also tried adding the following to application.conf
java.source=11.0.8
And for completeness java.source=14.0.1
When I run locally without Heroku (sbt compile stage) all is fine.
I have tried with the Play example from Heroku and also the Play example from Play (adding a procfile)... the result is the same.
The Play version is:
addSbtPlugin("com.typesafe.play" % "sbt-plugin" % "2.8.2")
I tried putting in some older versions but Heroku failed to compile.

Answer 1

I went back to trying to get the Heroku version to run locally (rather than the other way around). From the resulting error message I found that sbt installed via brew adds a dependency to JDK 13:
SBT fails with `String.class is broken`
I followed the fix listed there and installed sbt using SDKMAN (instructions here:
https://www.scala-sbt.org/1.x/docs/Installing-sbt-on-Mac.html)
This got me to the next error which was due to "old SBT version":
java.lang.NullPointerException in project root running sbt
After fixing this, I can now run Play on Heroku and locally