Remove 6.5.2, 8.0 and 8.0a9 from cdn.torproject.org

We are still keeping copies of 6.5.2 (for legacy/trac#19316 (comment 2220701)), 8.0 and 8.0a9 (for tor-browser#26050 (closed)) on cdn.torproject.org.

Since that's from 4 or 5 years ago, most users have probably already updated, or will never do it.

To save space, I think we can remove them. This will also make some space for #40678 (closed). We can also remove the rewrite rules in projects/release/update_responses_config.yml, or maybe make them return a 404 error so that the updater of those old versions will give an error.

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