doc: remove references to undepreciated commands being deprecated.

The dsa, ec, ecparam, and rsa manual pages refer to themselves are being
deprecated which they aren't.  Address this and add a note pointing to
the pkey command equivalents albeit without recommending it.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15239)