Update release automation so that 'push a tag' is the only action needed

[alexeagle]

Let's update this repo to match https://github.com/bazel-contrib/rules-template

The release automation is completely performed in a standard, shared GitHub Actions reusable workflow.

As rule maintainers, this reduces the possibility we can make a mistake during a release, and improves security by allowing GitHub Actions to fully verify the release artifacts. In fact, we'll be able to add some SLSA-style attestation to the bazel-contrib/rules-template in a couple weeks.

It also reduces the maintenance burden for the repo to the minimum necessary, which is important for its long term health as individual engineers get pulled into other things.