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/ZG3JNursG69dz1lr@paquier.xyz

src/interfaces/libpq/libpq-int.h

@@ -833,14 +833,8 @@ extern ssize_t pgtls_write(PGconn *conn, const void *ptr, size_t len);
  *
  * NULL is sent back to the caller in the event of an error, with an
  * error message for the caller to consume.
- *
- * This is not supported with old versions of OpenSSL that don't have
- * the X509_get_signature_nid() function.
  */
-#if defined(USE_OPENSSL) && (defined(HAVE_X509_GET_SIGNATURE_NID) || defined(HAVE_X509_GET_SIGNATURE_INFO))
-#define HAVE_PGTLS_GET_PEER_CERTIFICATE_HASH
 extern char *pgtls_get_peer_certificate_hash(PGconn *conn, size_t *len);
-#endif
 
 /*
  * Verify that the server certificate matches the host name we connected to.