GHA: use ubuntu-latest since ubuntu-20.04 has been retired.

Author: agarny

.github/workflows/release-docker-image.yml

@@ -8,7 +8,7 @@ on:
 jobs:
   release-docker-image:
     name: Release Docker image
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - name: Login to DockerHub
         uses: docker/login-action@v2