Remove Ubuntu 20.04 runners

Ubuntu 20.04 is no longer supported from GitHub actions, so this change
removes them to avoid failing our CI.

Signed-off-by: David Son <davbson@amazon.com>

Author: sondavidb

.github/workflows/build.yaml

@@ -13,7 +13,7 @@ jobs:
 
     strategy:
       matrix:
-        os: ['ubuntu-20.04', 'ubuntu-22.04']
+        os: ['ubuntu-22.04']
         go: ['1.23', '1.24']
       # Build all variants regardless of failures
       fail-fast: false