Remove Ubuntu 20.04 as build target due to EOL and ongoing maintance required (flameshot-org#3914)

Author: borgmanJeremy

.github/workflows/Linux-pack.yml

@@ -69,12 +69,6 @@ jobs:
               symbol: bookworm,
               arch: armhf
             }
-          - {
-              name: ubuntu-20.04,
-              os: ubuntu,
-              symbol: focal,
-              arch: amd64
-            }
           - {
               name: ubuntu-22.04,
               os: ubuntu,