Disable MK_OPENSSL_KTLS for stable/13.

Due to the pending release of 13.0 and the recent bug in serf exposed
by the KTLS changes, disable OpenSSL's KTLS by default. This keeps
crypto/openssl in sync with main while lowering the risk of introducing
instability into 13.0.

Discussed with: gjb (re), jkim, emaste (secteam)