dev/ci: add RunType for nightly release branch healthchecks
Created by: bobheadxi
Introduces a RunType that will trigger builds on the $major.$minor and $major.$minor-1 branches corresponding to the minimum upgradeable version.
The goal of this is to allow the Delivery and DevX team to keep tabs on the releasability of recent release branches to enable patches to be shipped faster - due to CI changes, patch releases seem to frequently run into blocking issues due to failed builds on outdated branches, for example: https://github.com/sourcegraph/sourcegraph/issues/30567, Slack discussion.
This RunType will be set up to be triggered via Buildkite scheduled builds once a day. Based on how it behaves, we can set up relevant alerting on it for #buildkite-main as well, though for now I suggest we manually monitor it.
This change is stacked on top of some pending refactors that improve the way RunTypes are implemented: https://github.com/sourcegraph/sourcegraph/pull/30581#issuecomment-1028371187
Example
