fix: Simplify CaC example step in README

[caitlynstocker]

What's this? 🐦

This simplifies the git_ref and git_commit inputs in our README example step for version-controlled projects.

The old examples contained fallbacks for tag and PR Git refs. Now that Octopus supports all types of Git refs, we can just use ${{ github.ref }} and ${{ github.sha }} and these will resolve to a usable ref no matter the type.

Completes DEVEX-374

[Copilot]

Pull request overview

This PR updates the README’s version-controlled project example to use the standard GitHub Actions context values for git reference and commit, simplifying the configuration for workflows triggered by branches, tags, or pull requests.

Changes:

• Replaced the previous git_ref expression with ${{ github.ref }} in the README example.
• Replaced the previous git_commit expression with ${{ github.sha }} in the README example and input documentation.

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