Add depcrecation warning for -m option

To get a client list, JSON RPC should be used instead of `--htmlstatus`/`-m`.
Check out https://github.com/jamulussoftware/jamulus/blob/main/docs/JSON-RPC.md for more information on JSON RPC.

Related to: #3362

Rephrase deprecation text

Co-authored-by: Peter L Jones <pljones@users.noreply.github.com>

src/main.cpp

@@ -342,6 +342,9 @@ int main ( int argc, char** argv )
         // HTML status file ----------------------------------------------------
         if ( GetStringArgument ( argc, argv, i, "-m", "--htmlstatus", strArgument ) )
         {
+            qWarning() << qUtf8Printable (
+                QString ( "- The HTML status file option (\"--htmlstatus\" or \"-m\") is deprecated and will be removed soon. Please use JSON-RPC "
+                          "instead.  See https://github.com/jamulussoftware/jamulus/blob/main/docs/JSON-RPC.md" ) );
             strHTMLStatusFileName = strArgument;
             qInfo() << qUtf8Printable ( QString ( "- HTML status file name: %1" ).arg ( strHTMLStatusFileName ) );
             CommandLineOptions << "--htmlstatus";
@@ -1111,7 +1114,7 @@ QString UsageArguments ( char** argv )
            "  -F, --fastupdate        use 64 samples frame size mode\n"
            "  -l, --log               enable logging, set file name\n"
            "  -L, --licence           show an agreement window before users can connect\n"
-           "  -m, --htmlstatus        enable HTML status file, set file name\n"
+           "  -m, --htmlstatus        deprecated, please use JSON-RPC instead\n"
            "  -o, --serverinfo        registration info for this Server.  Format:\n"
            "                          [name];[city];[country as two-letter ISO country code or Qt5 QLocale ID]\n"
            "      --serverpublicip    public IP address for this Server.  Needed when\n"