sha256: Fix minimum OpenSSL version

- Change the minimum OpenSSL version for using their SHA256
  implementation from 0.9.7 to 0.9.8.

EVP_sha256() does not appear in the OpenSSL source before 0.9.7h, and
does not get built by default until 0.9.8, so trying to use it for all
0.9.7 is wrong, and before 0.9.8 is unreliable.

Closes https://github.com/curl/curl/pull/8464

lib/sha256.c

@@ -40,7 +40,7 @@
 
 #include <openssl/opensslv.h>
 
-#if (OPENSSL_VERSION_NUMBER >= 0x0090700fL)
+#if (OPENSSL_VERSION_NUMBER >= 0x0090800fL)
 #define USE_OPENSSL_SHA256
 #endif