[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