Degrade 3DES to MEDIUM in SSL2

The SWEET32 fix moved 3DES from HIGH to MEDIUM, but omitted SSL2.

CLA: trivial
Reviewed-by: Kurt Roeckx <kurt@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1683)