Remove ubuntu-20.04 from horizon integration testing matrix (#5660)

Author: tamirms

.github/workflows/horizon.yml

@@ -10,7 +10,7 @@ jobs:
     name: Integration tests
     strategy:
       matrix:
-        os: [ubuntu-20.04, ubuntu-22.04]
+        os: [ubuntu-22.04]
         go: ["1.22", "1.23"]
         pg: [12, 16]
         protocol-version: [22]