Fill in release date for 2.13

CHANGELOG.txt

@@ -13,7 +13,7 @@ Changelog
  * Fix: The Wagtail admin urls will now respect the `APPEND_SLASH` setting (Tidjani Dia)
 
 
-2.13 (xx.xx.xxxx) - IN DEVELOPMENT
+2.13 (12.05.2021)
 ~~~~~~~~~~~~~~~~~
 
  * New StreamField implementation with performance and functionality improvements (Matt Westcott, Karl Hobley)

docs/releases/2.13.rst

@@ -1,6 +1,6 @@
-===========================================
-Wagtail 2.13 release notes - IN DEVELOPMENT
-===========================================
+==========================
+Wagtail 2.13 release notes
+==========================
 
 .. contents::
     :local: