Breaking changes
CancellationToken parameters are no longer nullable. They're still optional, so this should only affect code that was explicitly using CancellationToken?.
If you were previously passing in a nullable CancellationToken, I'd recommend null-coalescing to either default or CancellationToken.None.
If you were specifying the type in testing mocks, e.g. with Arg.Any<CancellationToken?>() or It.IsAny<CancellationToken?>(), simply remove the ?. If you were passing in null in tests, replace it with default or CancellationToken.None. If you were asserting that null was passed in as the cancellation token, it should be fine to assert on CancellationToken.None instead.