Add deploy readme.txt GitHub Action

[n7studios]

Summary

Adds a GitHub Action to only deploy the readme.txt file if changed in the main branch.

This allows changes to be made to the readme file (typically the Tested up to WordPress version number), without needing to deploy an entire version update to the Plugin.

Screenshots of this action tested on another Plugin, where the Tested up to value was changed to WordPress 6.3:

Testing

Existing tests pass.

Checklist

• [x] I have written a test and included it in this PR
• [x] I have run all tests and they pass
• [x] The code passes when running the PHP CodeSniffer
• [x] Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• [x] Security and Sanitization requirements have been followed
• [x] I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)