Upgrade the phpcs-diff action to use Node.js v20

[eason9487]

Changes proposed in this Pull Request:

Part of #108

This PR upgrades the phpcs-diff action to use Node.js v20

📌 Checklist before merging (@eason9487)

• Revert 3bba1ed before merging this PR

Detailed test instructions:

1. View a previous workflow run used v1 phpcs-diff action
   • https://github.com/woocommerce/automatewoo-birthdays/actions/runs/8699492678
   • There are warnings of using Node.js 16
     
2. View a test workflow run used updated phpcs-diff action
   • https://github.com/woocommerce/automatewoo-birthdays/actions/runs/8812893621?pr=134
   • This run failed on purpose to test if it can report PHP linting errors for the diff lines
   • There's no more warning of using Node.js 16
     
3. View the test PR
   • woocommerce/automatewoo-birthdays/pull/134
   • The annotations are shown
     
     

[mikkamp]

The changes look good to me.

I confirmed the warnings are there in previous runs, and no longer present in the latest run. I don't see any other references which need to be changed for this action.