How do I close all tickets before 2021?

Set up an automation for this.
You something along the lines of ticket created over 2200 hours ago, there may be an option around last agent comment as well.
Up to you if you do it on pending or open tickets.
Then you can set status = solve or closed.
I recommend you do a test first, perhaps add a tag test_ticket as a condition and then add this to just one ticket to prove the behaviour.