Remove 11.5.8 from cdn.tpo

During the Tor Browser weekly meeting, @pierov suggested that we remove old watershed versions from cdn.tpo and use archive.tpo URLs in update-responses for that version.

We can do this already for 11.5.8, which we have been keeping for a while on cdn.tpo. We'll need to update the xml files in tor-browser-update-responses.git/update_pre12.0/release to use archive.tpo URLs.

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