[sdk-platform-java] Migrate from Gradle to Maven

The goal of switching to Maven is to align with the rest of the Java client libraries repositories that use Maven. This should enable us to take advantage of more automation tools that have been designed and tested against Maven-based repositories in the googleapis organization. This should also simplify integration with Bazel and eliminate the need for `/dependencies.properties`.

- [x] Add Maven-based build without impacting Gradle and Bazel builds (#1562)
- [x] Verify that changing artifact parents is non-breaking
- [x] Verify that `mvn install` works and then using the newly installed version (x.x.x-SNAPSHOT) directly in any of the existing gapic clients (like java-language) still works.
- [x] Add Maven-based CI
- [x] Ensure that the Maven setup is aligned with other repos such as https://github.com/googleapis/java-core
- [x] Verify that all functions of the existing Gradle build configuration are covered
- [x] Switch CI from Gradle to Maven
- [x] Release using Maven
- [ ] Remove Gradle build configs
- [ ] Switch Bazel integration to the new Maven build instead of the existing Gradle one
- [ ] Discontinue reliance on `\dependencies.properties`
- - I don't think we can remove this file until we migrate gapic-generator-java to Maven