build: Modernize Gradle build infrastructure

[gsidhwani-nr]

Summary

• Upgrade verify plugin to 4.0, Gradle wrapper to 8.5
• Add/update gradle.properties with java.agent.version=8.6.0
• Update module build.gradle files to use project property for agent version
• Remove checkForDependencies task and newrelic-dependencies.sh
• Update release.yml: JDK 21, github-script@v7, permissions, clean install, comment out git push

Test plan

• ./gradlew clean install passes locally

Note: Please use squash merge to avoid changelog issues with merge commits.