Remove support for OpenSSL 1.0.1

Remove support for OpenSSL 1.0.1

Here are some notes about this change:
- As X509_get_signature_nid() should always exist (OpenSSL and
LibreSSL), hence HAVE_X509_GET_SIGNATURE_NID is now gone.
- OPENSSL_API_COMPAT is bumped to 0x10002000L.
- One comment related to 1.0.1e introduced by 74242c2 is removed.

Upstream OpenSSL still provides long-term support for 1.0.2 in a closed
fashion, so removing it is out of scope for a few years, at least.

Reviewed-by: Jacob Champion, Daniel Gustafsson
Discussion: https://postgr.es/m/[email protected]