Compatibility with PHP 8.1

[BenMorel]

Fix for a deprecation warning on PHP 8.1:

Return type of BackblazeB2\File::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

[BenMorel]

@mlambley Any chance this can be merged?

[mlambley]

Fixed via #78
Please confirm it now works.

[BenMorel]

This should work for PHP 8.0+, but I think you just broke compatibility with PHP 7.2 - 7.4 (the mixed return type is a new addition in PHP 8.0, while the #[ReturnTypeWillChange] is a way around the deprecation that is compatible with all versions).

It looks like your last Travis build ran 2 years ago, do you want a PR to add GitHub actions instead? This would prevent such issues.

[mlambley]

Yeah that's a good point. Please resolve your conflicts and I'll merge this instead.

[BenMorel]

@mlambley Done!