Fill in release date for 2.9

CHANGELOG.txt

@@ -25,7 +25,7 @@ Changelog
  * Fix: Accept unicode characters in slugs on the "copy page" form (François Poulain)
 
 
-2.9 (xx.xx.xxxx) - IN DEVELOPMENT
+2.9 (04.05.2020)
 ~~~~~~~~~~~~~~~~
 
  * Removed support for Django 2.1

docs/releases/2.9.rst

@@ -1,6 +1,6 @@
-==========================================
-Wagtail 2.9 release notes - IN DEVELOPMENT
-==========================================
+=========================
+Wagtail 2.9 release notes
+=========================
 
 .. contents::
     :local: