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

[noahdietz]

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.

[vchudnov-g]

Is it possible to link to/download release artifacts with wildcards? Because ideally we could still have the individual artifacts have the version in their names to be descriptive, but at the same time allow easily downloading latest. Hmmm....

[vchudnov-g]

And...no. Wildcards don't work. So here's a half-baked idea: I wonder whether with each release we could keep the versioned-name artifacts but create symlinks without the version name. That way we generally use the versioned-name one in some instructions/scripts that need to be pinned to a version number, but use the unversioned names where we need to, like for this issue.

I'm not sure that's the way we should go, but maybe. Maybe it's overkill. But it seems useful if the user could know which version they're getting when they download "latest". GitHub feature request? ;-)