release: create stub PRs for deploy-sourcegraph-docker, kubernetes upgrade

Created by: bobheadxi

This will simplify https://github.com/sourcegraph/about/pull/1838 by removing the need to add PRs to the campaign manually

Case in point:

Create a PR to update the Kubernetes upgrade guide indicating the steps required to upgrade. Add the created pull request to the release campaign:
```
yarn run release release:add-to-campaign $MAJOR.$MINOR.0 sourcegraph/sourcegraph <pr-number>
```
Create a PR to update deploy-sourcegraph-docker as required. Add the created pull request to the release campaign:
```
yarn run release release:add-to-campaign $MAJOR.$MINOR.0 sourcegraph/deploy-sourcegraph-docker <pr-number>
```

And some of the manual steps in https://github.com/sourcegraph/deploy-sourcegraph-docker/blob/master/RELEASING.md , that have automated equivalents for other deploy-sourcegraph repos

Many of these still have manual steps that go beyond what is trivial to automate, so these PRs should indicate follow-up action is required.