Add support for phpdocumentor/reflection-docblock v6

[mspirkov]

We really need support for phpdocumentor/reflection-docblock v6 in this version, as we need support for PHP 7.4. Is it possible to do this?

https://github.com/yiisoft/yii2-apidoc/blob/caeb69a9119b7e8fa38b913f04d034ad437eeba7/composer.json#L24-L27

It would also be great to support the sixth version of phpdocumentor/reflection-docblock in the sixth version of this package.

[mspirkov]

@jaapio Please take a look

[jaapio]

I think the result of this will be a breaking change in this library, So I think we need to create a new major version to make this possible.

[mspirkov]

Got it. Then should I close this PR?

[mspirkov]

And one more question. Do you have time to release new major version?

[jaapio]

I see you targeted the 5.x release. Most likely due to the 7.4 constraint. I do not remember exactly what the reason was back then to raise the constraints to 8.1, I think it was related to one of the other packages enforcing me to raise the php version.

So I doubt if we could bring php 7.4 back. I would like to avoid that anyway since php 7.4 is not supported anymore and support of many php versions in this package is hard.

I do have some time to help you with a new release when needed. I hope to have a bit more time to do work next month.