I'm not sure what is the problem. The announcements informs that you have to use Java 11+ to execute scans, but you can still compile your code with Java <11. You didn't provide any information about your project, so let's take a Maven as an example.
It generally means that you have to do something like this:
// set Java to 8
export JAVA_HOME=/path/to/jdk8/
// compile, test and build
mvn package
// set Java to 11
export JAVA_HOME=/path/to/jdk11/
// execute scanner
mvn sonar:sonar
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