Fix wrong value for visibility option in XSD

[sebastian-meyer]

The phpDocumentor command line helper says, that the parameter visibility allows the following values:

api, public, protected, private, internal.

But the configuration XSD enumerates the following values as valid options:

api, public, protected, private, hidden.

So internal is missing from the XSD and using hidden actually leads to an error.

This pull requests fixes this issue by replacing hidden with internal in the XSD.

(I already opened a pull request for this issue [#3663], but that was marked as "merged" although is wasn't.)

[sebastian-meyer]

The failing QA workflow reports an issue with a part of code this pull request doesn't change. Am I supposed to fix this anyway with this PR?

[jaapio]

It's ok, I will take care of psalm. If you want to fix it feel free to open a pr. That would be very helpful.

Thanks for addressing this!