Fill in release date for 2.12.4

CHANGELOG.txt

@@ -37,7 +37,7 @@ Changelog
  * Fix: Apply enough chevron padding to all applicable select elements (Scott Cranfill)
 
 
-2.12.4 (xx.xx.xxxx)
+2.12.4 (19.04.2021)
 ~~~~~~~~~~~~~~~~~~~
 
  * Fix: CVE-2021-29434 - fix improper validation of URLs ('Cross-site Scripting') in rich text fields (Kevin Breen, Matt Westcott)

docs/releases/2.12.4.rst

@@ -1,6 +1,6 @@
-=============================================
-Wagtail 2.12.4 release notes - IN DEVELOPMENT
-=============================================
+============================
+Wagtail 2.12.4 release notes
+============================
 
 .. contents::
     :local: