xfs: deprecate the ascii-ci feature

This feature is a mess -- the hash function has been broken for the
entire 15 years of its existence if you create names with extended ascii
bytes; metadump name obfuscation has silently failed for just as long;
and the feature clashes horribly with the UTF8 encodings that most
systems use today.  There is exactly one fstest for this feature.

In other words, this feature is crap.  Let's deprecate it now so we can
remove it from the codebase in 2030.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>