Enable email notification step by default on Helm (#2975)

# What this PR does

Set `oncall.smtp.enabled` to `true` by default to enable email
notifications on Helm deployments.
Email notifications are enabled by default on docker-compose deployments
already: see [this feature
flag](df6f6183ec/engine/settings/base.py (L63)).

## Which issue(s) this PR fixes

https://github.com/grafana/oncall/issues/2917

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- Enable email notification step by default on Helm by @vadimkerr ([#2975](https://github.com/grafana/oncall/pull/2975))
 - Handle slack resolution note errors consistently ([#2976](https://github.com/grafana/oncall/pull/2976))
 
 ## v1.3.35 (2023-09-05)

helm/oncall/values.yaml

@@ -198,7 +198,7 @@ oncall:
     # The key in the secret containing Telegram token
     tokenKey: ""
   smtp:
-    enabled: false
+    enabled: true
     host: ~
     port: ~
     username: ~