[DOCS] Add 6.8.21 release notes to index (#81655)

Author: jrodewig

docs/reference/release-notes.asciidoc

@@ -5,6 +5,7 @@
 --
 This section summarizes the changes in each release.
 
+* <<release-notes-6.8.21>>
 * <<release-notes-6.8.20>>
 * <<release-notes-6.8.19>>
 * <<release-notes-6.8.18>>