Update Sidekiq to v5.2.9

This contains only minor changes that tighten up some dependencies:

* https://github.com/mperham/sidekiq/blob/master/Changes.md
* https://github.com/mperham/sidekiq/compare/v5.2.7...v5.2.9

gitlab-exporter is using v5.2.9 already, so this change avoids duplicate
versions being shipped in Omnuibus.

Note we avoid v5.2.8 since this unnecessarily restricts Rack.

Gemfile.lock

@@ -1004,11 +1004,11 @@ GEM
     shellany (0.0.1)
     shoulda-matchers (4.0.1)
       activesupport (>= 4.2.0)
-    sidekiq (5.2.7)
+    sidekiq (5.2.9)
       connection_pool (~> 2.2, >= 2.2.2)
-      rack (>= 1.5.0)
+      rack (~> 2.0)
       rack-protection (>= 1.5.0)
-      redis (>= 3.3.5, < 5)
+      redis (>= 3.3.5, < 4.2)
     sidekiq-cron (1.0.4)
       fugit (~> 1.1)
       sidekiq (>= 4.2.1)

changelogs/unreleased/sh-update-sidekiq-5-2-9.yml

+---
+title: Update Sidekiq to v5.2.9
+merge_request: 35495
+author:
+type: other