Hey there,
Under the same drop down menu in the screenshot you provided, click the "NONE" option. This will get you back to the version supported by TMP
If you are having difficulties, here is a step by step guide:
1. Head to your "Game Library" on steam
2. Right click on ETS2
3. Click "Properties"
4. Click "Betas"
5. Under the drop down menu, click on "none". (the very first option)