How do I uninstall bitdefender?
Following an 8 year old tutorial installing some superfluent (if not harmful) piece of software is not a very good idea I guess ;-)
Fix by deleting the file created in the tutorial:
sudo rm /etc/apt/sources.list.d/bitdefender.list
Then you should be able to sudo apt update again.
Note, that some of your warnings are unrelated to that tutorial and won't go away but also won't be a problem (just warnings). Check here how to fix this.