Support PHP 8.3

What's Changed

• Support PHP 8.3 (#8)
• Bump actions/checkout from 3 to 4 (#7) Did you know you can use @dependabot to update your actions, not just your code? I've updated my article which mentions Dependabot https://www.michalspacek.com/dont-let-security-bugs-catch-you-off-guard#github-dependabot