chore/release: deprecation notice (#416)

closes PLAT-41

## Description

We plan to deprecate the kustomize deployment type. This PR adds a
warning notice to various easy to see places warning admins of the
deprecation plan.

---

## 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.
-->

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

## Test plan

<!--
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.
-->

Author: DaedalusG

README.md

@@ -1,5 +1,8 @@
 # Deploy Sourcegraph on Kubernetes with Kustomize
 
+> **⚠️ Deprecation Notice**
+> This Kustomize-based deployment method is planned for deprecation. We recommend using [Helm](https://sourcegraph.com/docs/admin/deploy/kubernetes) for Kubernetes deployments.
+
 [![sourcegraph: search](https://img.shields.io/badge/sourcegraph-search-brightgreen.svg)](https://sourcegraph.com/github.com/sourcegraph/deploy-sourcegraph-k8s) [![master build status](https://badge.buildkite.com/018ed23ed79d7297e7dd109b745597c58d875323fb06e81786.svg?branch=main)](https://buildkite.com/sourcegraph/deploy-sourcegraph-k8s)
 
 > **Note**

instances/README.md

@@ -1,5 +1,8 @@
 # Instances
 
+> **⚠️ Deprecation Notice**
+> This Kustomize-based deployment method is planned for deprecation. We recommend using [Helm](https://sourcegraph.com/docs/admin/deploy/kubernetes) for Kubernetes deployments.
+
 See the [Sourcegraph Kustomize docs](https://docs.sourcegraph.com/admin/deploy/kubernetes/kustomize) for the latested instructions.
 
 ## Create an overlay for your instances

instances/template/kustomization.template.yaml

@@ -1,4 +1,9 @@
 ##########################################################################################
+# ⚠️  DEPRECATION NOTICE
+# This Kustomize-based deployment method is planned for deprecation.
+# We recommend using Helm for Kubernetes deployments.
+# See: https://sourcegraph.com/docs/admin/deploy/kubernetes
+##########################################################################################
 # DEPLOY INSTRUCTIONS
 #
 # Build Manifests: kubectl kustomize instances/$CURRENT_DIR -o cluster.yaml