switch docs to reference "latest" release assets instead of specific version

We should change all of the examples in the README to use the "latest" release version, specifically when downloading release assets. See https://docs.github.com/en/repositories/releasing-projects-on-github/linking-to-releases.

As part of this, we'd need to exclude the release version from the asset names e.g. `gapic-showcase-0.24.0-darwin-amd64.tar.gz` --> `gapic-showcase-darwin-amd64.tar.gz`. This would involve changes to the release asset creation workflow and notification to downstream consumers that the naming scheme changed.