Hello @bufbooth, this community post should clarify your questions.
You are already running SonarQube with JDK17, that warning is related to the fact that your sonar scanner is using JDK11.
JDK11 for running the scanner was deprecated a few months ago and SonarQube 10.4 will drop the support for it.
The post I linked has all the details on the topic.
Regards,
Matteo