Migrate sre-oncall-usergroup-updater to woodhouse

Our current usergroup updating logic (that syncs PagerDuty escalation policies to the @sre-oncall slack handle) lives in the following unmaintained code bases:

• https://ops.gitlab.net/gitlab-com/gl-infra/infra-automation-commons/slack-wrapper
• https://ops.gitlab.net/gitlab-com/gl-infra/sre-oncall-usergroup-updater
• https://console.cloud.google.com/appengine/versions?serviceId=sre-oncall-usergroup-updater&project=gitlab-infra-automation

Consumers of the slack wrapper are: sre-oncall-usergroup-updater and release-tools (for @release-managers).

Those code bases are a pain to maintain, and due to them being unmaintained, they pose a security risk. We've migrated most of the other tools from that suite into woodhouse. This is quite possibly the last one. Once the migration is complete, we can decommission the legacy infra.

Migrating this to woodhouse will also give us more flexibility to introduce more of these slack aliases (@release-managers, @incident-managers, @cmoc).