Use sha256 as digest algorithm in certificate generator

This makes the generated certificates compatible with security
level 2, which is apparently the default on Ubuntu 20.04.

Unfortunately this does not fix all tests, because some are using
pre-generated certificates.

Author: nikic

ext/openssl/tests/CertificateGenerator.inc

@@ -97,7 +97,8 @@ class CertificateGenerator
             openssl_csr_new($dn, $this->lastKey, ['req_extensions' => 'v3_req']),
             $this->ca,
             $this->caKey,
-            2
+            /* days */ 2,
+            ['digest_alg' => 'sha256'],
         );
 
         $certText = '';