[DOCS] Add ml-cpp PRs to release notes (#60839)

Author: lcawl

docs/reference/release-notes.asciidoc

@@ -5,6 +5,7 @@
 --
 This section summarizes the changes in each release.
 
+* <<release-notes-6.8.12>>
 * <<release-notes-6.8.11>>
 * <<release-notes-6.8.10>>
 * <<release-notes-6.8.9>>

docs/reference/release-notes/6.8.asciidoc

@@ -1,3 +1,17 @@
+[[release-notes-6.8.12]]
+== {es} version 6.8.12
+
+coming::[6.8.12]
+
+Also see <<breaking-changes-6.8,Breaking changes in 6.8>>.
+
+[float]
+[[bug-6.8.12]]
+=== Bug fixes
+
+Machine Learning::
+* Fix restoration of change detectors after seasonality change {ml-pull}1391[#1391]
+
 [[release-notes-6.8.11]]
 == {es} version 6.8.11