ci: update config
ci: update config

- test against fewer go versions, only the last 2 major versions of
  Go are supported so we can run tests against only those two
- only test against version with 32bit
- remove the CircleCI module cache. With the new module proxy it is faster to
  download from the proxy.

Also refactor the config a bit to:

- remove duplication in the go-test jobs (we can specify GOARCH as a
  parameter)
- use the same executor for both jobs, so that the docker registry url
  can be specified in a single place
- remove the need for any yaml anchors and aliases

hashicorp/raft,hashicorp/raft