src: const_cast is necessary for 1.1.1, not 0.9.7

The const_cast used to be necessary for SSL_get_app_data() in OpenSSL
0.9.7, but node doesn't compile against OpenSSL versions that old.
However, now it's needed for the recently introduced
SSL_renegotiate_pending(), which is not const-correct as of 1.1.1a.

PR-URL: https://github.com/nodejs/node/pull/25861


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>