ci: no longer test MPS 2020.3 compatibility

We dropped official support for that MPS version.

Author: mhuster23

.github/workflows/mps-compatibility.yaml

@@ -16,7 +16,6 @@ jobs:
     strategy:
       matrix:
         version:
-          - "2020.3"
           - "2021.1"
           - "2021.2"
           - "2021.3"