Set published min JVM version to 8

[jkasten2]

• Fixes issue where required JVM was set to 11 from 0.13.2
  • Most likely com.gradle.plugin-publish:0.15.0 introduced the .module
    file. This defines a min JVM version which happens to default to 11.

This fixes errors such as:

 A problem occurred configuring project ':app'.
   > Could not resolve all artifacts for configuration ':app:classpath'.
      > Could not resolve gradle.plugin.com.onesignal:onesignal-gradle-plugin:[0.12.6, 0.99.99].
        Required by:
            project :app
         > No matching variant of gradle.plugin.com.onesignal:onesignal-gradle-plugin:0.13.2 was found. The consumer was configured to find a runtime of a component compatible with Java 8, packaged as a jar, and its dependencies declared externally but:
             - Variant 'apiElements' capability gradle.plugin.com.onesignal:onesignal-gradle-plugin:0.13.2 declares a component, packaged as a jar, and its dependencies declared externally:
                 - Incompatible because this component declares an API of a component compatible with Java 11 and the consumer needed a runtime of a component compatible with Java 8
             - Variant 'runtimeElements' capability gradle.plugin.com.onesignal:onesignal-gradle-plugin:0.13.2 declares a runtime of a component, packaged as a jar, and its dependencies declared externally:
                 - Incompatible because this component declares a component compatible with Java 11 and the consumer needed a component compatible with Java 8

---

This change is