Use Gradle 7 to build the project

[KengoTODA]

Hello, 👋

I want to suggest bumping up Gradle from 6 to 7, to enhance build performance by the configuration cache. Actually, there are fewer build performance enhancements, because this repo is enough small and simple for now.

This PR also contains several minor changes:

• Replace actions/cache with the built-in dependencies cache in actions/setup-java, which provides better performance.
• Replace deprecated task definition with suggested tasks.register() API, refer the GRadle official manual for detail.
• Create a settings.gradle file which is always suggested to have in Gradle projects.
• Bump up the org.sonarqube Gradle plugin to the latest release.
• Use --release option instead of targetCompatibility. It helps us to avoid depending on Java API which does not exists in Java 8.

Thanks for checking my PR! Hope that it helps your daily hacking.

[kazuma-t]

Thank you for your nice patch!