Move handover issue generation to woodhouse
### TLDR
We want to move the SRE handover issue generation into woodhouse.
### Background
The logic for the old handover generator (written in nodejs) is spread across multiple projects:
* https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9030
* https://ops.gitlab.net/gitlab-com/gl-infra/sre-oncall-handover-automation
* https://ops.gitlab.net/gitlab-com/gl-infra/infra-automation-commons/gitlab-gateway
* https://ops.gitlab.net/gitlab-com/gl-infra/infra-automation-commons/pagerduty-gateway
Those services are not well maintained, they're deployed to AppEngine, and have been a bit of a hassle to work with. By moving everything into woodhouse, we may be able to decommission some of that infrastructure and make it easier to add new features to the handover.
### Recommendation
Re-implement this logic in woodhouse, then decommission the old services.
### Proof of concept
https://gitlab.com/gitlab-com/gl-infra/woodhouse/-/merge_requests/132
### Rollout plan
- [x] Run it side-by-side with existing handover generator, posting to `gitlab-com/gl-infra/on-call-handovers-test`.
- [x] Once the reports look good, disable the cron job for the old handover generator, and point woodhouse at `on-call-handovers`.
- [ ] Wait a few days.
- [ ] Decommission legacy handover generator.
issue