feat(rel): add step to release to update main with latest version (#212)

## Description

Add step to build process that creates a PR to update `main` with the
latest release version.

---

## Checklist

<!--
Kubernetes, both Kustomize and Helm, and Docker Compose MUST be kept in
sync.
You should not merge a change here without a corresponding change in the
other repositories,
unless it is specific to this deployment type. If uneeded, add link or
explanation of why it is not needed here.
-->

- [NA] Update
[CHANGELOG.md](https://github.com/sourcegraph/deploy-sourcegraph-k8s/blob/main/CHANGELOG.md)
- [NA] Update [K8s Upgrade
notes](https://github.com/sourcegraph/sourcegraph/blob/main/doc/admin/updates/kubernetes.md)
- [NA] Kustomiz-specific changes
- [NA] Update sister repository:
[deploy-sourcegraph-helm](https://github.com/sourcegraph/deploy-sourcegraph-helm)
- [NA] Update sister repository:
[deploy-sourcegraph-docker](https://github.com/sourcegraph/deploy-sourcegraph-docker)
- [NA] Verify all images have a valid tag and SHA256 sum

## Test plan

CI during release cycle.

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As part of SOC2/GN-104 and SOC2/GN-105 requirements, all pull requests
are REQUIRED to
provide a "test plan". A test plan is a loose explanation of what you
have done or
implemented to test this, as outlined in our Testing principles and
guidelines:

https://docs.sourcegraph.com/dev/background-information/testing_principles
  Write your test plan here after the "Test plan" header.
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Author: jdpleiness

release.yaml

@@ -332,6 +332,7 @@ promoteToPublic:
           cat << EOF | buildkite-agent annotate --style info
           Promoted release is **publicly available** through a git tag at [\`{{version}}\`](https://github.com/sourcegraph/deploy-sourcegraph-k8s/tree/{{version}}).
           EOF
+
       - name: 'Promote on release registry'
         cmd: |
           echo "Promoting deploy-sourcegraph-k8s {{version}} release on release registry"
@@ -348,3 +349,61 @@ promoteToPublic:
             echo "Build created, see:"
             echo $body | jq .web_url
           fi
+
+      - name: "update main with latest version"
+        cmd: |
+          set -eu
+          branch="promote/release-{{version}}-update-main"
+          echo "Checking out origin/main"
+          git fetch origin main
+          git switch main
+          echo "Creating branch origin/${branch}"
+          git switch -c "${branch}"
+
+      - name: "sg ops"
+        cmd: |
+          set -eu
+          sg ops update-images \
+            --kind k8s \
+            --registry index.docker.io/sourcegraph \
+            --docker-username=$DOCKER_USERNAME \
+            --docker-password=$DOCKER_PASSWORD \
+            --pin-tag {{inputs.server.tag}} \
+            base/
+
+      - name: "sg ops (executors)"
+        cmd: |
+          set -eu
+          sg ops update-images \
+            --kind k8s \
+            --registry index.docker.io/sourcegraph \
+            --docker-username=$DOCKER_USERNAME \
+            --docker-password=$DOCKER_PASSWORD \
+            --pin-tag {{inputs.server.tag}} \
+            components/executors/
+
+      - name: "git:commit"
+        cmd: |
+          set -eu
+          branch="promote/release-{{version}}-update-main"
+          find . -name "*.yaml" | xargs git add
+          find . -name "*.yml" | xargs git add
+
+          # Careful with the quoting for the config, using double quotes will lead
+          # to the shell dropping out all quotes from the json, leading to failed
+          # parsing.
+          git commit -am 'update-main: {{version}}' -m 'update main with latest release'
+          git push origin "${branch}"
+          
+      - name: "github:pr"
+        cmd: |
+          set -eu
+          internal_branch="promote/release-{{version}}-update-main"
+          gh pr create \
+            --fill \
+            --draft \
+            --base "$internal_branch" \
+            --title "Update main: build {{version}}" \
+            --body "Test plan: automated release PR, CI will perform additional checks"
+          echo "🚢 Please check the associated CI build to ensure the process completed".
+