[mlir] Switch default Fold API to using FoldAdaptors

As mentioned in https://discourse.llvm.org/t/psa-new-improved-fold-method-signature-has-landed-please-update-your-downstream-projects/67618, it has now been about one month since the new fold API has been introduced.

Any downstream users that have not yet switched, nor explicitly set useFoldAPI to kEmitRawAttributesFolder will experience a build break after this patch, requiring them to either switch to the new API or explicitly use the old one and live with the warning set by TableGen.

Differential Revision: https://reviews.llvm.org/D143497