Fill in release date for 4.2.1

CHANGELOG.txt

@@ -75,7 +75,7 @@ Changelog
  * Maintenance: Move snippet choosers and model check registration to `SnippetViewSet.on_register()` (Sage Abdullah)
 
 
-4.2.1 (xx.xx.xxxx) - IN DEVELOPMENT
+4.2.1 (13.03.2023)
 ~~~~~~~~~~~~~~~~~~
 
  * Fix: Support creating `StructValue` copies (Tidiane Dia)

docs/releases/4.2.1.md

@@ -1,6 +1,6 @@
-# Wagtail 4.2.1 release notes - IN DEVELOPMENT
+# Wagtail 4.2.1 release notes
 
-_Unreleased_
+_March 13, 2023_
 
 ```{contents}
 ---