To make sure this question has an answer on GD:SE to help others when searching I'm posting the link you mentioned:
I've recently ran into this exact same problem and I've found 2 ways to solve my issue, so I'm hoping I can help you.
This post is becoming aged but keeps popping up so I'll add an edit.
Since the original post I've been made aware of an easier way to mark
your animation files as Legacy:
Project window, then go to the top right corner of the Inspector
window and select Debug from the drop down menu (next to the lock),
then change the Animation Type to 1 will mark it as Legacy.
I'm not sure if you did, but when I created my original animation
files, I did so inside of the "Project" window. Try adding an
animation component to the sprite you want to animate, then open the
animation window and select "[create new animation]" in the drop down
menu. Doing it this way seems to create the new animation with the
legacy animation flag set. This way you can completely avoid the
warning or need for editing animation files. I hope this solves your
issue, I know how frustrating it was for me.