Upgrade Github Actions' runner image

`ubuntu-20.04` is getting deprecated.

See actions/runner-images#11101
See manastech/infra#99

Author: matiasgarciaisaia

.github/workflows/ci.yml

@@ -4,9 +4,9 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Generate images' Dockerfiles
         run: ./dockerfiles.sh
       - name: Build Docker images