chore: add Helm chart considerations to upgrade guide (#862)

Author: Katie Horne

setup/upgrade/considerations.md

@@ -32,3 +32,46 @@ upgrading, such as architecture updates and breaking changes.
 
 - v1.25 updates the username format to allow the use of alphanumeric character
   and hyphens. The length of the username can be 1-39 characters, inclusive.
+
+## Upgrading from versions prior to v1.21
+
+Users upgrading deployments that predate the release of v1.21 to v1.21 or later
+should update their Helm values file to reflect Coder's [updated schema]. More
+specifically, users must change the following values:
+
+- `cemanager` --> `coderd`
+- `cemanager.replicas` --> `coderd.replicas`
+- `cemanager.image` --> `coderd.image`
+- `cemanager.resources` --> `coderd.resources`
+- `devurls.host` --> `coderd.devurlsHost`
+- `ingress.loadBalancerIP` --> `coderd.serviceSpec.loadBalancerIP`
+- `ingress.loadBalancerSourceRanges` -->
+  `coderd.serviceSpec.loadBalancerSourceRanges`
+- `ingress.service.externalTrafficPolicy` -->
+  `coderd.serviceSpec.externalTrafficPolicy`
+- `ingress.tls.hostSecretName` --> `coderd.tls.hostSecretName`
+- `ingress.tls.devurlsHostSecretName` --> `coderd.tls.devurlsHostSecretName`
+- `storageClassName` --> `postgres.default.storageClassName`
+- `timescale.image` --> `postgres.default.image`
+- `timescale.resources` --> `postgres.default.resources`
+- `timescale.resources.requests.storage` -->
+  `postgres.default.resources.requests.storage`
+- `postgres.useDefault` --> `postgres.default.enable`
+- `deploymentAnnotations` --> `services.annotations`
+- `serviceTolerations` --> `services.tolerations`
+- `clusterDomainSuffix` --> `services.clusterDomainSuffix`
+- `serviceType` --> `services.type`
+- `serviceAccount.annotations` -->
+  `coderd.builtinProviderServiceAccount.annotations`
+- `serviceAccount.labels` --> `coderd.builtinProviderServiceAccount.labels`
+
+<!-- Turn off linting to avoid changing the link -->
+<!-- markdownlint-disable MD044 -->
+
+[updated schema]:
+  https://github.com/coder/enterprise-helm/blob/1.27.0/values.yaml
+
+<!-- markdownlint-enable MD044 -->
+
+> The Helm charts shipped with versions 1.21 through 1.26 are
+> backward-compatible, while charts shipping with v1.27 and later are not.