Set org.gradle.tooling.parallel where org.gradle.parallel is enabled

Gradle 9.8 deprecates relying on org.gradle.parallel=true to also turn on parallel model building for Tooling API clients such as IDEs, and says this will fail in Gradle 10. Gradle 9.4 added org.gradle.tooling.parallel to control it. This recipe adds org.gradle.tooling.parallel=true, which keeps today's behavior, to a gradle.properties that enables org.gradle.parallel and sets no explicit value. It never creates a file, and leaves a file alone where parallel execution is disabled or absent. See the Gradle upgrade guide for more information.