provide a "moving" v5 tag

[StephenWithPH]

It would be convenient for users of your GitHub Actions to have a "moving" vMajor tag. That allows them--for example--to always use the latest v5.x.x by specifying uses: buildpacks/github-actions/setup-pack@v5.

Note that this aligns with the guidance here: https://docs.github.com/en/actions/creating-actions/releasing-and-maintaining-actions#example-developer-process

We recommend creating releases using semantically versioned tags – for example, v1.1.3 – and keeping major (v1) and minor (v1.1) tags current to the latest appropriate commit.

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Currently, only full x.y.z tags exist, so users have to pin to the specific version and periodically bump their tags: