Increase finder-frontend unicorn workers
Increase finder-frontend unicorn workers

This will allow us to at least halve the number of calculators_frontend machines
currently running. It's probable that we'll be able to switch the remaining
ones to a general purpose compute type, as [load is negligible](https://grafana.production.govuk.digital/dashboard/file/machine.json?refresh=1m&orgId=1&var-hostname=calculators_frontend-ip-10-13-4-152_eu-west-1_compute_internal&var-cpmetrics=All&var-filesystem=All&var-disk=All&var-tcpconnslocal=All&var-tcpconnsremote=All&from=now-2d&to=now)
at present.

It's a bit of a faff recreating instances, so I'd prefer to destroy some
first to make the remaining job smaller.

The other apps on this machine type are licence-finder and smart-answers which
haven't had their unicorn workers changed since the days of 3 frontend machines
so I'm leaving them alone.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet