Fill in release date for 5.1

CHANGELOG.txt

@@ -1,7 +1,7 @@
 Changelog
 =========
 
-5.1 (xx.xx.xxxx) - IN DEVELOPMENT
+5.1 (01.08.2023)
 ~~~~~~~~~~~~~~~~
 
  * Add support for read-only FieldPanels (Andy Babic)

docs/releases/5.1.md

@@ -1,6 +1,6 @@
-# Wagtail 5.1 release notes - IN DEVELOPMENT
+# Wagtail 5.1 release notes
 
-_Unreleased_
+_August 1, 2023_
 
 ```{contents}
 ---