GH Actions: update for new version of "Create Pull Request"

The "Create Pull Request" action has released a new major version.

I have verified that the workflow as-is would still work.

However, this new version also offers one relevant new feature:
* Signing commits.
    The PRs created by this actions are created by the `github-actions[bot]` user. Adding the `sign-commits: true` option will allow the bot to sign the commits based on the repo default `GITHUB_TOKEN`.

Refs:
* https://github.com/peter-evans/create-pull-request/releases/tag/v7.0.0
* https://github.com/peter-evans/create-pull-request/blob/v7.0.0/docs/concepts-guidelines.md#commit-signature-verification-for-bots

.github/workflows/happy-new-year.yml

@@ -78,6 +78,7 @@ jobs:
           base: ${{ steps.branches.outputs.BASE }}
           branch: ${{ steps.branches.outputs.PR_BRANCH }}
           delete-branch: true
+          sign-commits: true
           commit-message: "Squiz/FileComment: update year in test case fixed file"
           title: "Squiz/FileComment: update year in test case fixed file"
           # yamllint disable rule:line-length