[ci/cloud-deploy] Remove CI success requirement (elastic#141965)

This removes the success requirement in the cloud deployment step for
pull requests.

Creating a cloud deployment from a pull request is optional and not
providing test coverage.  Failures are typically upstream and not
something that can be fixed from the Kibana repository.

.buildkite/pipelines/pull_request/deploy_cloud.yml

@@ -5,6 +5,7 @@ steps:
       queue: n2-2-spot
     depends_on: build
     timeout_in_minutes: 30
+    soft_fail: true
     retry:
       automatic:
         - exit_status: '-1'