PHP 8.5 support and some maintenance

What's Changed

• Start testing on PHP 8.5 (#349)
• Add a link to a new Symfony config package that has just dd() and dump() for now (#353)

Internal changes

• Update Dead Code Detector (#350)
• Remove nikic/php-parser as a direct dev dependency (#347)
• Support nikic/php-parser 5.6.1 in tests (#348)
• Bump actions/checkout from 4 to 5 (#345)
• Narrow the thrown type in docblock to void when no exception is thrown (#352)