coap_openssl.c: Support OpenSSL 3.0+ client talking to legacy server

OpenSSL 3.0+ has removed the SSL_OP_LEGACY_SERVER_CONNECT option from the
default set list meaning that a OpenSSL 3.0+ client is unable to set up a
session with a TinyDTLS server.

This fix adds back in SSL_OP_LEGACY_SERVER_CONNECT to the set list of
options.

This removes the message that reports the failure
"unsafe legacy renegotiation disabled"
that causes the session set up to fail.

There is debate as to whether RFC 5746 is appropriate for DTLS sessions,
which is what OpenSSL 3.0+ now enforces unless SSL_OP_LEGACY_SERVER_CONNECT
is set.

src/coap_openssl.c

@@ -786,6 +786,9 @@ void *coap_dtls_new_context(coap_context_t *coap_context) {
     SSL_CTX_set_cookie_verify_cb(context->dtls.ctx, coap_dtls_verify_cookie);
     SSL_CTX_set_info_callback(context->dtls.ctx, coap_dtls_info_callback);
     SSL_CTX_set_options(context->dtls.ctx, SSL_OP_NO_QUERY_MTU);
+#if OPENSSL_VERSION_NUMBER >= 0x30000000L
+    SSL_CTX_set_options(context->dtls.ctx, SSL_OP_LEGACY_SERVER_CONNECT );
+#endif /* OPENSSL_VERSION_NUMBER >= 0x30000000L */
     context->dtls.meth = BIO_meth_new(BIO_TYPE_DGRAM, "coapdgram");
     if (!context->dtls.meth)
       goto error;