Allow Danger step to fail

Currently the Danger processing is producing false positives when master is
merged into a branch. Until #49175 is fixed, we should allow it to fail.

.gitlab-ci.yml

@@ -351,6 +351,7 @@ retrieve-tests-metadata:
 danger-review:
   image: registry.gitlab.com/gitlab-org/gitaly/dangercontainer:latest
   stage: prepare
+  allow_failure: true
   before_script:
     - source scripts/utils.sh
     - retry gem install danger --no-ri --no-rdoc