Update ECS docs to version 8.9 (#2725)

Author: kilfoyle

shared/versions/stack/8.9.asciidoc

@@ -13,7 +13,7 @@ bare_version never includes -alpha or -beta
 :prev-major-version:     7.x
 :prev-major-last:        7.17
 :major-version-only:     8
-:ecs_version:            master
+:ecs_version:            8.9
 :esf_version:            master
 
 //////////