Bump Markdown cache

We removed vulnerability references in Markdown so we must bump the
cache so that we won't have these links to vulnerabilities in the HTML
cache.

This also caused problems with reference redaction because the
vulnerability parser class no longer exists.

lib/gitlab/markdown_cache.rb

@@ -3,7 +3,7 @@
 module Gitlab
   module MarkdownCache
     # Increment this number every time the renderer changes its output
-    CACHE_COMMONMARK_VERSION        = 25
+    CACHE_COMMONMARK_VERSION        = 26
     CACHE_COMMONMARK_VERSION_START  = 10
 
     BaseError = Class.new(StandardError)