Reflect deprecated-status of bridgedb

There are places in our spec that refer to bridgedb. But IIUC bridgedb is obsolete, and has been replaced by rdsys.

We can mark as historical the parts of our spec that say "here is how bridgedb works." But for the parts of our system that explain how other pieces of Tor interact with bridgedb, I think we should instead refer to "the bridge distributor" or "a bridge distributor".

cc @tpo/anti-censorship

added Backlog For Anticensorship Team labels

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I think it makes sense to talk generically as "the bridge distributor".

We could deprecate the bridgedb spec. Most of this document covers the architecture or bridgedb, that kind of documentation for rdsys now lives in rdsys repo. The only parts of this document that are still valid are "Displaying Bridge Information", which describes the format of a bridge line, and "Writing bridge assignments for statistics", which we'll deprecate once the metrics team is ready to use the prometheus metrics we produce.

Not sure what will be the right place to put the bridge line format. We could make a document about it together with the url format that we are working on: tpo/anti-censorship/team#126

assigned to @nickm

added Next label and removed Backlog label

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mentioned in merge request !350 (merged)

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mentioned in commit fa055e1b

closed with merge request !350 (merged)