[5.0.x] Added stub release notes and release date for 5.0.8 and 4.2.15.

Backport of 3f88089 from main.

Author: sarahboyce

docs/releases/4.2.15.txt

@@ -2,9 +2,10 @@
 Django 4.2.15 release notes
 ===========================
 
-*Expected August 6, 2024*
+*August 6, 2024*
 
-Django 4.2.15 fixes a regression in 4.2.14.
+Django 4.2.15 fixes three security issues with severity "moderate", one
+security issue with severity "high", and a regression in 4.2.14.
 
 Bugfixes
 ========

docs/releases/5.0.8.txt

@@ -2,9 +2,10 @@
 Django 5.0.8 release notes
 ==========================
 
-*Expected August 6, 2024*
+*August 6, 2024*
 
-Django 5.0.8 fixes several bugs in 5.0.7.
+Django 5.0.8 fixes three security issues with severity "moderate", one security
+issue with severity "high", and several bugs in 5.0.7.
 
 Bugfixes
 ========