Bump the CACHE_COMMONMARK_VERSION

Since we needed to bump the version to 13 in the backports,
and we know that an MR on master also bumped it
to 13, bump to 14 to ensure that when a customer
upgrades to the most recent release, the markdown
gets recalcuated as necessary.

app/models/concerns/cache_markdown_field.rb

@@ -15,7 +15,7 @@ module CacheMarkdownField
   # Increment this number every time the renderer changes its output
   CACHE_REDCARPET_VERSION         = 3
   CACHE_COMMONMARK_VERSION_START  = 10
-  CACHE_COMMONMARK_VERSION        = 13
+  CACHE_COMMONMARK_VERSION        = 14
 
   # changes to these attributes cause the cache to be invalidates
   INVALIDATED_BY = %w[author project].freeze