Add a check to ensure we have a valid PHP version before loading plugin functionality and output an admin notice if needed

[kmgalanakis]

Description of the Change

In this PR, I'm adding a check for the minimum required PHP version before attempting to load the plugin files. If the condition is not met an admin notice is displayed and the plugin files are not loaded.

Closes https://github.com/10up/simple-page-ordering/issues/150

How to test the Change

• To see the admin notice and have the plugin functionality not loaded, try to load the plugin on a site with PHP version lower that 7.4.

Changelog Entry

Added - Check for minimum required PHP version before loading the plugin

Credits

Props @kmgalanakis

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests pass. c