Improve return types to prevent deprecation warnings

[zluiten]

What changed

• UpcastingIterator::key() gets a native mixed return type, replacing the #[\ReturnTypeWillChange] attribute and @return mixed docblock.
• MergedStreamIterator::current() and UpcastingIterator::current() retain their existing signatures - see below for why.
• prooph/bookdown-template is removed from require-dev as it requires league/commonmark ~0.7 which only resolves to 0.19.x-dev, incompatible with PHP 8+. This was breaking the entire CI pipeline.

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UpcastingIterator::key() is safe because UpcastingIterator is final - no subclasses can exist, so adding a return type cannot break anyone.

The other two cannot use native types without a BC break:

• MergedStreamIterator::current() - MergedStreamIterator is not final. Adding any return type to a previously untyped method forces subclasses to declare a compatible type or receive a deprecation warning.
• UpcastingIterator::current() - already has ?Message. Widening it to mixed is flagged as non-covariant by the BC checker, as callers may rely on the narrower type. Since ?Message already satisfies Iterator::current(): mixed covariantly, no deprecation warning is generated and no change is needed.

[zluiten]

@basz Can you have look?