Add PHP 8 compatibility

[jeffpaul]

The plugin currently does not set a "Requires PHP", so we'll want to work on testing and adding compatibility for PHP 8 so that we can declare that more formally within the code.

• add PHP Compatibility action: https://github.com/10up/insert-special-characters/blob/develop/.github/workflows/php-compatibility.yml
• add PHP 8.3 compatibility (or a lessor version as required by the codebase or Mailchimp constraints)
• add "Requires PHP" header value with associated minimum to the main PHP file
• add "PHP tested up to" header value with associated known maximum to the main PHP file
• update

  wordpress/composer.json

  Line 15 in 3dd04ab

  "php": ">=5.6"

  to note PHP minimum

[dkotter]

My recommendation here would be the following:

1. Setup a local environment running PHP 7.4 and run through all plugin functionality, both in the admin and front-end. Take note of any warnings, errors, deprecation notices, etc that get logged and fix those
2. Update that environment to PHP 8.3 and run through all tests again, making note of any new warnings, errors, deprecations, etc that get logged, fixing those

A good majority of users are still on PHP 7.4 so I think it's worth the small amount of extra effort to test on 7.4 and 8.3. In addition, while we run PHPCompat on most of our plugins it doesn't actually support PHP 8.x+ yet, so that's nice to add but worth noting that we still need to run manually testing to verify PHP 8.x+ compat.

[dkotter]

Talking with Mailchimp today, we are going to set PHP 7.0 as our minimum. The hope is to fairly quickly bump that to 7.4 but wanted to have smaller bumps rather than one large one