doc: Add a hint to man3/EVP_$hash that it is legacy

doc: Add a hint to man3/EVP_$hash that it is legacy

Some hash algorithms are only provided by the legacy provider. This
information is not mentioned in EVP_md4(3) for md4 and one might wonder
why it is no longer working.

Add a note to the EVP_ man page for md2, md4, mdc2, ripemd160 and
whirlpool that it is only available with the legacy provider.

Fixes #11650

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/12074)