Fill in release date for 2.16

CHANGELOG.txt

@@ -9,7 +9,7 @@ Changelog
 * Change comment headers’ date formatting to use browser APIs instead of requiring a library (LB (Ben Johnston))
 
 
-2.16 (xx.xx.xxxx) - IN DEVELOPMENT
+2.16 (07.02.2022)
 ~~~~~~~~~~~~~~~~~
 
  * Added support for Django 4.0

docs/releases/2.16.md

@@ -1,4 +1,4 @@
-# Wagtail 2.16 release notes - IN DEVELOPMENT
+# Wagtail 2.16 release notes
 
 ```eval_rst
 .. contents::