Serverless QA quality gate - soft-fail Fleet smoke tests (#220902)

## Summary

This PR sets the Fleet smoke tests in the serverless release QA quality
gates pipeline to soft-fail.

Every now and then, we see flaky behavior of the Fleet smoke test
pipeline. Having it on hard-fail means, that the underlying argo
workflow also fails, so we have to re-submit the workflow. Setting it to
soft-fail allows us to just re-run the Buildkite pipeline (or assess
otherwise that we're good to continue the release) and still continue
with the argo workflow. At this stage, the risk of this change is low
because a release manager is manually reviewing the results of the QA
quality gates before unlocking promotion to staging.

.buildkite/pipelines/quality-gates/pipeline.tests-qa.yaml

@@ -24,6 +24,7 @@ steps:
 
   - label: ":ship: Fleet serverless smoke tests for ${ENVIRONMENT}"
     trigger: fleet-smoke-tests # https://buildkite.com/elastic/fleet-smoke-tests
+    soft_fail: true
     build:
       env:
         ENVIRONMENT: ${ENVIRONMENT}