Install JDK + Gradle

AGP 8.6+ requires JDK 21. AGP 8.0-8.5 works with 17. RN 0.83+ ships AGP 8.6.

Install JDK 21 (LTS)

# macOS — Zulu (free, Azul builds)
brew install --cask zulu@21

# Or Temurin (Eclipse Adoptium)
brew install --cask temurin@21

# Verify
java -version           # → 21.x
javac -version          # → 21.x

# /usr/libexec/java_home on macOS shows install path
/usr/libexec/java_home -V