[CI] Update docker_keyserver workflow to enable autoscaling

Summary: Set `agent` to `autoscaling` to enable autoscaling. Also shut down the last actively managed CI instance on AWS.. so now everything except for the iOS build is "autoscalling."

Test Plan: Buildkite CI

Reviewers: def-au1t, palys-swm, yayabosh, ashoat

Reviewed By: ashoat

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4410

.buildkite/docker_keyserver.yml

@@ -2,4 +2,4 @@ steps:
   - command:
       - 'cd keyserver && touch .env && bash/dc.sh build --no-cache'
     agents:
-      - 'docker=true'
+      - 'autoscaling=true'