Fix `Navigator.pushNamed<T>()` type checking (#188148)

This PR allows the navigator's `pushNamed()` method to be called with
any type argument without immediately throwing an error.
It also brings the framework more in line with the [style
guide](https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md#prefer-explicit-types-and-avoid-dynamic):

> Avoid `dynamic`. If the type is unknown, prefer using `Object` (or
`Object?`) and casting

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Fixes #57186