Simplify DispatchData.append(_: UnsafeBufferPointer<T>)
Simplify DispatchData.append(_: UnsafeBufferPointer<T>)

The deprecation warning guides us towards UnsafeRawBufferPointer,
so sure, let's use that.

No intended functionality change.

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch