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.

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

(cherry picked from commit 3cf25a7802a26ae3be9159c585fda6aea3d0dc08)