Fix PHP 8.4 deprecation warning

[hemberger]

Implicitly nullable parameters (i.e. those accepting null without a null typehint) are deprecated in PHP 8.4. Marking $library as explicitly nullable fixes the following deprecation warning:

Deprecated: Nbbc\BBCode::__construct(): Implicitly marking parameter $library as nullable is deprecated, the explicit nullable type must be used instead

Explicitly nullable types were not added until PHP 7.1, so the minimum requirement is raised from 5.6 (which reached EOL in 2017) to 7.1 (which was released in 2016). Documentation has been updated accordingly.

Added PHP 8.3 and 8.4 to the GitHub Actions test matrix.