Prepare CHANGELOG for 8.12.1 & 8.13.0

[ci skip]
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 Please view this file on the master branch, on stable branches it's out of date.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 8.12.0 (unreleased)
+v 8.13.0 (unreleased)
+
+v 8.12.1 (unreleased)
+
+v 8.12.0
   - Update the rouge gem to 2.0.6, which adds highlighting support for JSX, Prometheus, and others. !6251
   - Only check :can_resolve permission if the note is resolvable
   - Bump fog-aws to v0.11.0 to support ap-south-1 region