add backport notice

Katie Horne · Katie Horne · commit d37e1df2d108 · 2021-09-13T17:07:14.000-05:00
diff --git a/changelog/1.21.3.md b/changelog/1.21.3.md
@@ -13,6 +13,8 @@ There are no new features in 1.21.3.
 
 ### Bug fixes 🐛
 
+> Coder v1.21.3 includes bug fixes backported from v1.22.1.
+
 - infra: Fixed goroutine/memory leaks.
 
 ### Security updates 🔐
