[mlir] Fix -Wdeprecated-declarations in BufferResultsToOutParams.cpp (NFC)

/llvm-project/mlir/lib/Dialect/Bufferization/Transforms/BufferResultsToOutParams.cpp:124:26:
error: 'cast' is deprecated: Use mlir::cast<U>() instead [-Werror,-Wdeprecated-declarations]
  124 |           orig.getType().cast<MemRefType>().hasStaticShape()) {
      |