Update CHANGELOG for 8.8.6 and 8.7.8

[ci skip]

CHANGELOG

@@ -223,6 +223,10 @@ v 8.9.0
   - Add tooltip to pin/unpin navbar
   - Add new sub nav style to Wiki and Graphs sub navigation
 
+v 8.8.6
+  - Fix visibility of snippets when searching.
+  - Update omniauth-saml to 1.6.0 !4951
+
 v 8.8.5
   - Import GitHub repositories respecting the API rate limit !4166
   - Fix todos page throwing errors when you have a project pending deletion !4300
@@ -353,6 +357,10 @@ v 8.8.0
   - When creating a .gitignore file a dropdown with templates will be provided
   - Shows the issue/MR list search/filter form and corrects the mobile styling for guest users. #17562
 
+v 8.7.8
+  - Fix visibility of snippets when searching.
+  - Update omniauth-saml to 1.6.0 !4951
+
 v 8.7.7
   - Fix import by `Any Git URL` broken if the URL contains a space
   - Prevent unauthorized access to other projects build traces