chore: add changelog for 1.21.4

Katie Horne · Katie Horne · commit dbf908775877 · 2021-09-22T15:01:52.000-05:00
diff --git a/manifest.json b/manifest.json
@@ -423,6 +423,9 @@
         {
           "path": "./changelog/1.22.0.md"
         },
+        {
+          "path": "./changelog/1.21.4.md"
+        },
         {
           "path": "./changelog/1.21.3.md"
         },
