build(ci): use JetBrains JDK

Now we've moved to Gradle Daemon JVM Criteria, 21/JetBrains
will be downloaded if it doesn't exist on disk

Given that Temurin and JetBrains are effectively equivalent, in a headless build,
standarddizing it saves us a wasted download

Issue 20382

Assisted-by: Claude Opus 4.7 & 4.8