[diag] quick fix to support long output (#3436)

`Cli::Uart::OutputFormat()` current only supports output
`OPENTHREAD_CONFIG_CLI_MAX_LINE_LENGTH` bytes, which isn't enough
for command *diag* by default. This PR removes that restriction
by using `Cli::Uart::Output`.

I think this is just a quick fix, an overall enhancement of Cli is
needed so that we can use small stack size but also get better
performance.

src/cli/cli.cpp

@@ -3742,7 +3742,7 @@ void Interpreter::ProcessDiag(int argc, char *argv[])
     // all diagnostics related features are processed within diagnostics module
     output[sizeof(output) - 1] = '\0';
     otDiagProcessCmd(argc, argv, output, sizeof(output) - 1);
-    mServer->OutputFormat("%s\n", output);
+    mServer->Output(output, static_cast<uint16_t>(strlen(output)));
 }
 #endif