Update to Java 25

[kahgoh]

Java 25 is the new LTS release. These are some notes to update to Java 25:

• Update test runner. Dependabot had previously update to Java 25, but there were CI errors. See Bump eclipse-temurin from 21 to 25 java-test-runner#164.
• Check docs for references to Java 21, such as INSTALLATION.
• Update concepts (for example flexible constructor bodies
• Check if any of the exercises also need to be updated (e.g. if builds for the exercises need updating).