Add CHANGELOG for 8.12.10

Signed-off-by: Rémy Coutable <remy@rymai.me>

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
+v 8.12.10
+  - Fix information disclosure in `Projects::BlobController#update`
+  - Fix missing access checks on issue lookup using IssuableFinder
+  - Replace issue access checks with use of IssuableFinder
+
 v 8.12.9
   - Fix XSS issue in Markdown autolinker.
 

changelogs/unreleased/jej-22869.yml

----
-title: Fix information disclosure in `Projects::BlobController#update`
-merge_request: 
-author: 

changelogs/unreleased/jej-fix-missing-access-check-on-issues.yml

----
-title: Fix missing access checks on issue lookup using IssuableFinder
-merge_request: 
-author: 

changelogs/unreleased/jej-use-issuable-finder-instead-of-access-check.yml

----
-title: Replace issue access checks with use of IssuableFinder
-merge_request: 
-author: 

changelogs/unreleased/markdown-xss-fix-option-2-1.yml

----
-title: Fix XSS issue in Markdown autolinker
-merge_request: 
-author: