nghttpx: Set default minimum TLS version to TLSv1.2

Previously, the default minimum TLS version was TLSv1.1, but the
default cipher list didn't include any compatible ciphers with it.
This made handshake fail if TLSv1.1 was negotiated because there was
no shared ciphers.  To make the default settings consistent, the
default minimum TLS version is now TLSv1.2.

src/shrpx.cc

@@ -1392,7 +1392,7 @@ constexpr auto DEFAULT_NPN_LIST = StringRef::from_lit("h2,h2-16,h2-14,"
 } // namespace
 
 namespace {
-constexpr auto DEFAULT_TLS_MIN_PROTO_VERSION = StringRef::from_lit("TLSv1.1");
+constexpr auto DEFAULT_TLS_MIN_PROTO_VERSION = StringRef::from_lit("TLSv1.2");
 #ifdef TLS1_3_VERSION
 constexpr auto DEFAULT_TLS_MAX_PROTO_VERSION = StringRef::from_lit("TLSv1.3");
 #else  // !TLS1_3_VERSION
@@ -2131,7 +2131,11 @@ SSL/TLS:
               --tls-min-proto-version and  --tls-max-proto-version are
               enabled.  If the protocol list advertised by client does
               not  overlap  this range,  you  will  receive the  error
-              message "unknown protocol".  The available versions are:
+              message "unknown protocol".  If a protocol version lower
+              than TLSv1.2 is specified, make sure that the compatible
+              ciphers are  included in --ciphers option.   The default
+              cipher  list  only   includes  ciphers  compatible  with
+              TLSv1.2 or above.  The available versions are:
               )"
 #ifdef TLS1_3_VERSION
                              "TLSv1.3, "