Clean up redirect files

Add expiry dates to redirects that were missing
them, and delete redirects more than a year old.

doc/topics/airgap/index.md

----
-redirect_to: '../offline/index.md'
----

doc/user/admin_area/analytics/instance_statistics.md

@@ -3,3 +3,6 @@ redirect_to: 'usage_trends.md'
 ---
 This document was moved to [another location](usage_trends.md).
+
+<!-- This redirect file can be deleted after <2022-03-20>. -->
+<!-- Before deletion, see: https://docs.gitlab.com/ee/development/documentation/#move-or-rename-a-page -->

doc/user/clusters/agent/runner.md

@@ -4,5 +4,5 @@ redirect_to: 'https://docs.gitlab.com/runner/install/kubernetes-agent.html'
 This document was moved to [another location](https://docs.gitlab.com/runner/install/kubernetes-agent.html).
-<!-- This redirect file can be deleted after <2021-04-22>. -->
+<!-- This redirect file can be deleted after <2022-02-01>. -->
 <!-- Before deletion, see: https://docs.gitlab.com/ee/development/documentation/#move-or-rename-a-page -->
\ No newline at end of file