Java Gradle build: Modernize build scripts, update deps and address deprecations Update build scripts in preparation for Gradle 9: - Use Task Configuration Avoidance API (`tasks.register` and `configureEach`) - Replace deprecated `mainClassName` property with `application.mainClass` - Use explicit property assignment for repository URLs - Update dependencies to later versions PiperOrigin-RevId: 966659125 Change-Id: Ic2bad8c9c231bfe79c5fd3f16205920edb8db48c