Deprecate stats.html and stats/*.csv files

With #25382 (moved) soon being merged, we should consider whether we still want to provide:

• specifications of current CSV files as well as
• CSV files like https://metrics.torproject.org/stats/servers.csv.

The main reason for removing them from the public interface is to reduce future maintenance effort. These files will still exist and we will still use them internally, we'd just not make them available for download anymore. The reduced maintenance effort mainly comes from not having to write similar specifications when adding new pre-aggregated CSV files.

Note that the data will still be available via the "Download data as CSV." links on all graph pages. See #25382 (moved) for that.

What reasons might exist for leaving these CSV files still available. Are people using them and cannot switch easily to the new, ad-hoc generated CSV files?

If we decide to take these files down, we'll have to give users a reasonable heads-up. 1--2 months seems reasonable. We should announce that decision at least on stats.html and possibly in other places, too.