Update release tool to version helm chart the same as other deployment repos
Created by: caugustus-sourcegraph
Standardize on release/x.y
as the release branch name for helm charts. We're also going to keep the version and appVersion in sync.
Test plan
Ran a dry-run batch change and confirmed that both sourcegraph and sourcegraph-migrator Chart.yaml's were updated and the helm docs were re-generated (no more manual steps).