[DOCS] Add placeholder for 6.8.16 release notes (#73202)

Author: jrodewig

docs/reference/release-notes.asciidoc

@@ -5,6 +5,7 @@
 --
 This section summarizes the changes in each release.
 
+* <<release-notes-6.8.16>>
 * <<release-notes-6.8.15>>
 * <<release-notes-6.8.14>>
 * <<release-notes-6.8.13>>

docs/reference/release-notes/6.8.asciidoc

@@ -1,3 +1,10 @@
+[[release-notes-6.8.16]]
+== {es} version 6.8.16
+
+coming::[6.8.16]
+
+Also see <<breaking-changes-6.8,Breaking changes in 6.8>>.
+
 [[release-notes-6.8.15]]
 == {es} version 6.8.15