Breaking change: simulateTransaction always returns replacementBlockhash

[mcintyre94]

Currently our type is:

type SimulateTransactionApiResponseWithReplacementBlockhash = Readonly<{
    /**
     * The blockhash that was used to simulate the transaction when `replaceRecentBlockhash` is
     * `true`.
     */
    replacementBlockhash: TransactionBlockhashLifetime;
}>;

In Agave 3.x replacementBlockhash is unconditionally included in the response, but is set to null when replaceRecentBlockhash was false.

We should update this type but it would be a breaking change in Kit, so pushing to a future PR.

See #1544 and e319fcc for full context