Revert "Disable MK_OPENSSL_KTLS for stable/13."

No further issues with the KTLS support in OpenSSL have been seen
since the serf/apache bugs which motivated disabling KTLS. In
addition, the KTLS API in OpenSSL is now finalized since it has been
released in OpenSSL 3.0.

This reverts commit 3cf25a7802a26ae3be9159c585fda6aea3d0dc08.

Approved by: jkim (maintainer), emaste
Sponsored by: Netflix