Merge branch 'upgrade_warning' into 'master'

Add upgrade warning for Consul

See merge request gitlab-com/www-gitlab-com!35478

data/release_posts/unreleased/upgrade-warning.yml

+# UPGRADE WARNING
+upgrade:
+  reporter: ljlane
+  description: |
+    The version of Consul in Omnibus GitLab was updated from 0.9.0 to 1.6.1 in GitLab 12.5. When upgrading Consul nodes in a high availability cluster, the Consul nodes must be upgraded one node at a time. The upgrade procedure is documented in [Upgrades for bundled Consul](https://docs.gitlab.com/ee/administration/high_availability/consul#upgrades-for-bundled-consul).