I assume, you are running the game on windows. Do not run the minecraft.exe file. minecraft.exe has a hardcoded bootstrap version number (5 I believe) and launcher will show the warning you mentioned for any bootstrap version below 100 if your OS is windows 8 and above.
create a shortcut to
Rename your existing
minecraft.dllso you don't run it again. this way you can keep the icon too.
modify shortcut you created in item 1 and change its icon to
minecraft.dllcreated in item 2
run the game using the new shortcut and message will not showup
There is no way to disable it, it is a warning that the application is outdated and not recommended anymore. It may become unstable later on, so upgrading is a good idea. While the new launcher might not be your favorite, it does everything the old one can and more.