Upgrade the automerge-released-trunk, coverage-report, and prepare-php actions to use Node.js v20

[eason9487]

Changes proposed in this Pull Request:

Part of #108

This PR upgrades 3 actions that mainly use external actions:

• automerge-released-trunk
• coverage-report
• prepare-php

Detailed test instructions:

💡 I don't prepare a test for the automerge-released-trunk action as it has only an update for actions/checkout and it should work well.

1. View a previous workflow run used v1 coverage-report and prepare-php
   • https://github.com/woocommerce/automatewoo/actions/runs/8756576318
   • There are warnings of using Node.js 16
     
2. View a test workflow run used updated coverage-report and prepare-php
   • https://github.com/woocommerce/automatewoo/actions/runs/8799042429?pr=1740
   • There's no more warning related to the actions in this PR
     
3. View the test PR
   • woocommerce/automatewoo/pull/1740
   • The Coverage report was commented correclty.
     

[eason9487]

I will be skipping the code review process as it can be optional for a devs-facing only change.