fix(gradle-build): include maven plugin
fix(gradle-build): include maven plugin

This PR includes (adds) the maven plugin. This plugin is currently
deprecated:

```text
Deprecated Gradle features were used in this build, making it
incompatible with Gradle 7.0.

Use '--warning-mode all' to show the individual deprecation
warnings.
```

```text
The maven plugin has been deprecated. This is scheduled to be removed
in Gradle 7.0. Please use the maven-publish plugin instead. Consult
the [upgrading guide for further information](https://docs.gradle.org/6.3/userguide/upgrading_version_5.html#legacy_publication_system_is_deprecated_and_replaced_with_the_publish_plugins)
```

A more helpful link might be [this one here](https://docs.gradle.org/6.3/userguide/publishing_maven.html#sec:modifying_the_generated_pom).

For now, we are including `maven` alongside `maven-publish`.

imgix/imgix-java