Set CI Java version to match AMI Java version (11)

I'm not sure why I set the CI Java version to 17 (other than I have
started using that Java version elsewhere) - but as the AMI is only
`editorial-tools-focal-java11-ARM-WITH-cdk-base`, when the server
tried to run code that had been compiled with Java 17, it gave an
UnsupportedClassVersionError like this in the logs:

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 61.0), this version of the Java Runtime only recognizes class file versions up to 55.0]
	at play.api.http.HttpErrorHandlerExceptions$.throwableToUsefulException(HttpErrorHandler.scala:384)

.github/workflows/ci.yml

@@ -27,7 +27,7 @@ jobs:
       uses: actions/setup-java@v4
       with:
         distribution: corretto
-        java-version: 17
+        java-version: 11
         cache: sbt
     - name: CI yarn
       shell: bash