Switch to Java 11 and add Java 20 support

[lukfor]

This PR updates to Java 11 and should address the following issues that are related to Java

• nf-test script should respect JAVA_HOME and JAVA_CMD environment variables #163
  • integrate the same logic and variables as in Nextflow? https://github.com/nextflow-io/nextflow/blob/master/nextflow#L263
• General error during conversion: Unsupported class file major version 64 #152
• Use Java 11, 17 and 21 github actions