add changelog for release 1.21.4

Author: Katie Horne

changelog/1.21.4.md

@@ -0,0 +1,24 @@
+---
+title: "1.21.4"
+description: "Released on 09/22/2021"
+---
+
+### Breaking changes ❗
+
+There are no breaking changes in 1.21.4.
+
+### Features ✨
+
+- infra: updated Helm service account and RBAC objects to be kept when
+  uninstalling Helm.
+
+### Bug fixes 🐛
+
+- web: fixed issue where users had to rebuild workspaces twice after
+  regenerating SSH keys.
+- infra: fixed race condition seen during the "starting networking agent" build
+  step leading to a "context deadline exceeded" error message.
+
+### Security updates 🔐
+
+There are no security updates in 1.21.4.

manifest.json

@@ -441,6 +441,9 @@
         {
           "path": "./changelog/1.22.0.md"
         },
+        {
+          "path": "./changelog/1.21.4.md"
+        },
         {
           "path": "./changelog/1.21.3.md"
         },