Fix java upgrade docs

docs/java.md

@@ -2,10 +2,10 @@
 
 First, consider sticking to LTS unless there's a very desired feature or an important bug fix. There's often a gap between _no support_ of a non-LTS version and Gradle not supporting the newer one yet.
 
-- make sure there's an OpenJDK release (https://openjdk.org/)
+- make sure there's a Temurin release (https://adoptium.net/temurin/releases/)
 - make sure IntelliJ supports it (https://www.jetbrains.com/help/idea/supported-java-versions.html although this page is often wrong)
 - make sure Gradle supports it in its toolchain mode (its normal mode is hopeless since they started adding Kotlin, also Kotlin frequently lags behind JDK releases)
-- download the latest _Oracle OpenJDK_ release in IntelliJ
+- download the latest _Temurin_ release in IntelliJ
 - switch the project SDK to it
 - change the java toolchain version in the root build.gradle
 - run all tests and make a dry run