Add Markdown cache version upgrades dashboard

Adds a dashboard to watch the Markdown cache version upgrade counter. (No test dashboard since it's a new one.)

See https://docs.gitlab.com/development/gitlab_flavored_markdown/banzai_pipeline_and_parsing/#rollout-procedure-for-a-cache_commonmark_version-bump for documentation on the cache bump process; note steps 3 and 4 currently just name the counter, but you still have to go into Grafana Explore and write a query yourself. Let's improve this!