Fixes PHP 8.4 deprecation warnings

[freost]

Fixes the "Implicitly marking parameter ... as nullable is deprecated" deprecation warnings

[tacman]

Any downsides will accepting this PR and publishing a new version?

[nadar]

PHP 8.4 adoption is growing, so it would be nice to have support for it, @cebe . I obviously have the same issue with PHP 8.5 as well.

[nadar]

Maybe if you don’t have time (which is totally understandable, since you’re spending your free time on open source), you could add another maintainer to help? There seems to be many open PR's and Issues.

[tacman]

Another maintainer would be better than a fork.

If B/C is an issue, what about releasing a 2.0 with only PHP 8.4+ and the latest of the libraries? A quick rector pass would also make the code more maintainable because static analysis is so much better with the later versions.

[nadar]

@cebe is there anything we could do here to help you? or to help maintain the repo? PHP 8.4 als ready the most installed version currently, see => https://packagist.org/php-statistics

[nadar]

@tacman i have published this fork, if you need this fixed: https://github.com/nadar/cebe-php-openapi/releases/tag/1.0

replace cebe/php-openapi with nadar/cebe-php-openapi in your composer.json and adjust to version ^1.0.