Use the GitLab version as part of the appearances cache key

This prevents us from having to manually flush the cache key
in migrations every time a new column is added.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/5571

app/models/appearance.rb

@@ -16,7 +16,7 @@ class Appearance < ActiveRecord::Base
   has_many :uploads, as: :model, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
-  CACHE_KEY = 'current_appearance'.freeze
+  CACHE_KEY = "current_appearance:#{Gitlab::VERSION}".freeze
   after_commit :flush_redis_cache

changelogs/unreleased/sh-appearance-cache-key-version.yml

+---
+title: Use the GitLab version as part of the appearances cache key
+merge_request:
+author:
+type: fixed