do not run draft release workflow on branches named v2-<something>

[sophiewigmore]

Summary

It's common for us to create branches called v2-feature-x, causing pushes to that branch to get picked up by our release automation accidentally. This should resolve that.

With this change, notice that the Create Draft release workflow never ran for this branch v2-update-workflow with these changes - https://github.com/paketo-buildpacks/packit/actions/workflows/create-draft-release.yml?query=branch%3Av2-update-workflow

Use Cases

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).