Document deprecation of version-specific SSL/TLS methods

In commit 2b8fa1d5

 the version-specific SSL/TLS methods were
deprecated. This patch improves the documentation of that change
by stating the deprecation more prominently in the manual page
and explaining the reason for the deprecation.

Fixes #8989

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9001)