Disabling CI build for Ubuntu 20.04

Ubuntu 20.04 runner images are planned to be fully unsupported starting from [2025-04-15](actions/runner-images#11101), however they already produce run fails, due to unavailability.

Author: mcserep

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       matrix:
         db: [postgresql, sqlite3]
-        os: [ubuntu-20.04, ubuntu-22.04]
+        os: [ubuntu-22.04] # ubuntu-20.04
       fail-fast: false
 
     runs-on: ${{ matrix.os }}