Make default version explicit

[amckinney]

Now that we've gone through the exercise of releasing buf and all of the GitHub Actions to v1, we've realized that the implicit latest behavior is actually dangerous to use as a default value (which was introduced in #5).

If any breaking changes occur, the latest buf version might be incompatible with previous versions of the GitHub Actions. We can still give users the experience of "default to the latest version if you don't explicitly specify one", but it can instead be controlled by the buf release process so that buf-setup-action and buf are always released at the same time.

With all that said, users will still have access to the latest configuration value so that they can use a newer buf version than their version of buf-setup-action specifies. In this case, they can pin to an earlier version of buf-setup-action, but install the latest buf release, regardless of the action.yml's default value like so:

steps:
  - uses: actions/checkout@v2
  - uses: bufbuild/buf-setup-action@v0.4.0
    with:
      version: 'latest'

[doriable]

So the approach here would be "maxing out the latest" to 0.56.0, for example, right? E.g. without upgrading the setup action, you're basically restricted to this maximum version, but users can explicitly set the version input to latest to go beyond this version, right? I am okay with this user experience, so I am going to stamp this, but we may want to call out setting latest explicitly as a supported behaviour in the README.

[amckinney]

Yea I agree that we should update the README.md with the ability to set latest. I'll include a note here.

[amckinney]

Also to be clear, users aren't "maxed out" by the setting - they can still explicitly set a version that's higher than 0.56.0.