chore(release): deprecate 1.21.x releases (#2111)

Author: fieldju

_changelogs/1.21.5-changelog.md

@@ -2,7 +2,7 @@
 title: Version 1.21
 changelog_title: Version 1.21.5
 date: 2020-12-07 23:09:05 +0000
-tags: changelogs 1.21
+tags: changelogs 1.21 deprecated
 version: 1.21.5
 ---
 <script src="https://gist.github.com/spinnaker-release/98c3bab183b507662a8f5524e54626d4.js?file=1.21.5.md"></script>