Upgrade to PHP 8.3

[asgrim]

Now we can deploy PHP 8.3 in Heroku: heroku/heroku-buildpack-php#673 🎉

[asgrim]

Note; seeing this deprecated warning:

PHP Deprecated:  LazyPDO\LazyPDO implements the Serializable interface, which is deprecated. Implement __serialize() and __unserialize() instead (or in addition, if support for old PHP versions is necessary) in /app/vendor/lazypdo/lazypdo/src/LazyPDO.php on line 12

This should be fixed too.