Fill in release date for 5.2.1

CHANGELOG.txt

@@ -1,7 +1,7 @@
 Changelog
 =========
 
-5.2.1 (xx.xx.xxxx) - IN DEVELOPMENT
+5.2.1 (16.11.2023)
 ~~~~~~~~~~~~~~~~~~
 
  * Fix: Add a fallback background for the editing preview iframe for sites without a background (Ian Price)

docs/releases/5.2.1.md

@@ -1,6 +1,6 @@
-# Wagtail 5.2.1 release notes - IN DEVELOPMENT
+# Wagtail 5.2.1 release notes
 
-_Unreleased_
+_November 16, 2023_
 
 ```{contents}
 ---