Replace ArrayType::castToArrayKey() with Type::toArrayKey()

[rvanvelzen]

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[ondrejmirtes]

All aboard the Type refactoring train! 🚆 I can't wait to get rid of inheritance in Type classes.

[rvanvelzen]

The improvement actually causes some tests to fail legitimately, which is why I kept is as a draft. I'll improve those separately first so this can land cleanly :)

[herndlm]

ah right, I introduced usage of the old method again in the meantime in HasOffsetValueType, sorry 😊

[phpstan-bot]

This pull request has been marked as ready for review.

[ondrejmirtes]

Thank you!