Add a PHP version check and disable functionality if a site doesn't meet requirements

[dkotter]

Description of the Change

In the last release we bumped the minimum PHP version to 8.0. What we've found in the past is that sometimes sites are able to bypass the restrictions WordPress has in place around minimum versions and end up getting fatal errors when their site doesn't meet the requirements (for example, they may be running PHP 7.4 and somehow are able to install the latest version of the plugin which requires PHP 8.0, resulting in a fatal error).

This PR helps resolve that by adding a check before we load any of our plugin functionality. If a site doesn't meet our minimum requirements (in this case, PHP < 8.0) we output an admin notice and we don't load any more functionality.

Closes #125

How to test the Change

1. Checkout the develop branch and run npm run build-release
2. Create an environment running PHP 8.0+
3. Install and activate the plugin and note that things work
4. Downgrade the PHP version on that environment to 7.4
5. Note a fatal error is encountered
6. Checkout this branch and run npm run build-release again
7. Ensure the plugin shows an admin notice when running PHP 7.4 and works as expected with PHP 8.0+

Changelog Entry

Added - More robust minimum PHP version check.

Credits

Props @dkotter

Checklist:

• 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.

[dkotter]

@ravinderk Hoping to get a 1.2.1 release out here soon that contains that trailing comma fix. I think this would be good to go out with it as well so hoping you have a few minutes to review.

[ravinderk]

@dkotter This pull request is looking good.

Addition Request -
Can we set the PHP version to 8.0 in the plugin header information and readme.txt?

convert-to-blocks/convert-to-blocks.php

Line 8 in 255b459

* Requires PHP: 8

convert-to-blocks/readme.txt

Line 6 in 255b459

Requires PHP: 7.4