Add PHP 8.3 to testing matrix

[ttomdewit]

Summary

Since 8.3 is released I figured I'd add it to the testing matrix.

Explanation

Ran the following commands:

1. docker run --rm -v "$(pwd):/app" -w /app php:8.3-fpm-alpine ./vendor/bin/php-cs-fixer fix -v
2. docker run --rm -v "$(pwd):/app" -w /app php:8.3-fpm-alpine ./vendor/bin/phpunit
3. docker run --rm -v "$(pwd):/app" -w /app php:8.3-fpm-alpine ./vendor/bin/phpstan

All against 8.3 runtime, no errors (aside from PHP CS Fixer deprecations since #38 isn't merged yet).

Checklist

• I have provided a summary and an explanation
• I have reviewed the PR myself and left comments to provide context
• I have covered the changes with tests as appropriate - N/A
• I have made sure static analysis and other checks are successful

[osteel]

Thank you!