Fix API warning for older SDKs (#5337)

Summary:
Fixes warnings when building on macOS Sequoia with an older iOS SDK, e.g
```
executorch/backends/apple/mps/runtime/operations/QuantDequant.mm:35:60: error: 'MPSDataTypeInt4' is only available on iOS 18.0 or newer [-Werror,-Wunguarded-availability-new]
[CONTEXT]    35 |                                                   dataType:MPSDataTypeInt4];
[CONTEXT]       |                                                            ^~~~~~~~~~~~~~~
executorch/backends/apple/mps/runtime/operations/QuantDequant.mm:37:42: error: 'dequantizeTensor:scaleTensor:zeroPointTensor:dataType:name:' is only available on iOS 18.0 or newer [-Werror,-Wunguarded-availability-new]
[CONTEXT]    37 |   MPSGraphTensor *wDqTensor = [_mpsGraph dequantizeTensor:inputTensor
[CONTEXT]       |                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

cc shoumikhin, cccclai

Pull Request resolved: https://github.com/pytorch/executorch/pull/5337

Reviewed By: kirklandsign

Differential Revision: D62621256

Pulled By: shoumikhin

fbshipit-source-id: e651480734047e3ef2b76fb8333304e414ed21a1