Support Java 21 on CI

[mossmana]

While attempting to build the Flutter IntelliJ plugin for IntelliJ 2025.1, the version of Java needed to be bumped from 17 to 21. Build is successful when run locally. This version also needs to be configured on the CI. Refer to ./tool/github.sh and ./tool/kokoro/setup.sh

• What went wrong:
  Execution failed for task ':flutter-idea:compileJava'.
  > error: invalid source release: 21

#7963

[mossmana]

Related: #7967

[mossmana]

Investigation - Added logging to ./tool/github.sh (https://github.com/flutter/flutter-intellij/pull/7963/files#diff-b69f7145ef91717b5eec7e21d57643d051b1dd3e78ab95bacce2529bf43abfa2):

ls /usr/lib/jvm
temurin-11-jdk-amd64
temurin-1[7](https://github.com/flutter/flutter-intellij/actions/runs/13822518954/job/38670978251?pr=7963#step:3:8)-jdk-amd64
temurin-21-jdk-amd64
temurin-[8](https://github.com/flutter/flutter-intellij/actions/runs/13822518954/job/38670978251?pr=7963#step:3:9)-jdk-amd64
System Java version:
openjdk 17.0.14 2025-01-21
OpenJDK Runtime Environment Temurin-17.0.14+7 (build 17.0.14+7)
OpenJDK 64-Bit Server VM Temurin-17.0.14+7 (build 17.0.[14](https://github.com/flutter/flutter-intellij/actions/runs/13822518954/job/38670978251?pr=7963#step:3:15)+7, mixed mode, sharing)

Solution - set the JAVA_HOME and PATH environment variable to an available JDK 21 compatible version.
export JAVA_HOME=/usr/lib/jvm/temurin-21-jdk-amd64
...
export PATH=/usr/lib/jvm/temurin-21-jdk-amd64/bin:$PATH
...

java --version
openjdk 21.0.6 2025-01-21 LTS
OpenJDK Runtime Environment Temurin-21.0.6+7 (build 21.0.6+7-LTS)
OpenJDK 64-Bit Server VM Temurin-21.0.6+7 (build 21.0.6+7-LTS, mixed mode, sharing)

See: https://github.com/flutter/flutter-intellij/actions/runs/13822946487/job/38672266390?pr=7963