Add release dates to 2.15.6 / 2.16.3 release notes

docs/releases/2.15.6.rst

@@ -2,6 +2,8 @@
 Wagtail 2.15.6 release notes
 ============================
 
+*September 5, 2022*
+
 .. contents::
     :local:
     :depth: 1

docs/releases/2.16.3.md

@@ -1,5 +1,7 @@
 # Wagtail 2.16.3 release notes
 
+_September 5, 2022_
+
 ```eval_rst
 .. contents::
     :local: