dillon1000/react

Remove unstable_changedBits (#20953)

We added this unstable feature a few years ago, as a way to opt out of
context updates, but it didn't prove useful in practice.

We have other proposals for how to address the same problem, like
context selectors.

Since it was prefixed with `unstable_`, we should be able to remove it
without consequence. The hook API already warned if you used it.

Even if someone is using it somewhere, it's meant to be an optimization
only, so if they are using the API properly, it should not have any
semantic impact.