Increase gorouters in `prod` from 3 to 6
Increase gorouters in `prod` from 3 to 6

A tenant intends to run performance testing with about 600 requests ]
per second (RPS.) They will be doing this in our `prod` environment,
which receives about 300 RPS at present.

Going from 300 to 900 RPS will stress-test our request handling. This
commit doubles the number of gorouters that we run (VMs that process
and route incoming requests.)

During performance testing each gorouter should be processing about
150 requests per second, compared to 100 now. Hopefully this will give
us a better sense of how taxed our gorouters are and whether we need
to scale up like this in future.

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