VOTE: Rename OPENSSL_CTX to OSSL_LIB_CTX (as proposed by pull request #12621)

[Dr. Matthias St. Pierre]

The main rationale behind this change is consistency, because many of the new
OpenSSL 3.0 types have an OSSL_ prefix, and OPENSSL_CTX is a notable exception.
More details can be found in the description and thread of pull request #12621.

There was a discussion on openssl-committers and an initial poll on doodle about the
favourite replacements for OPENSSL_CTX (https://doodle.com/poll/drku9ziwvkp6tw25).

Matthias