Release v1.20.1

This updates the Envoy API definitions to v1.20.1. This release is downloadable from Maven Central.

v1.15.0

We've switched release revisions!

The project was previously tracking the same versions as https://github.com/envoyproxy/java-control-plane, but it's been slow to update the API recently.
Since this project is just taking the proto definitions and compiling them with ScalaPB without anything more, we've added scripts to fetch the proto definitions from the https://github.com/envoyproxy/envoy project instead, and are tracking against their version.

Moving forward, we'll use the Envoy version for releases, and if we need to publish a bugfix (e.g a compiler upgrade), we'll add a -<bugfix version> to the release.