disable kibana serverless validation

there are some space issues in the runners and we don't build kibana serverless anymore

.github/workflows/test-kibana-docker-image-action.yml

@@ -27,7 +27,10 @@ jobs:
       matrix:
         include:
           - serverless: false
-          - serverless: true
+          # Serverless is not something we built anymore as part of the obs automation
+          # but use the existing docker images generated by Kibana CI.
+          # See https://github.com/elastic/kibana/pull/181851
+          # - serverless: true
     steps:
       - name: Free Disk Space (Ubuntu)
         uses: jlumbroso/free-disk-space@54081f138730dfa15788a46383842cd2f914a1be
@@ -38,7 +41,6 @@ jobs:
           haskell: true
           large-packages: true
           docker-images: true
-
           # This needs to be set to false.
           # See https://github.com/jlumbroso/free-disk-space/issues/12
           # TL;DR