Bump sidekiq from 6.5.5 to 6.5.12

This also downgrades `redis`, since support for Redis 5.x was removed
from sidekiq in
sidekiq/sidekiq@d566154.

Gemfile.lock

@@ -647,10 +647,7 @@ GEM
       ffi (~> 1.0)
     rdoc (6.6.0)
       psych (>= 4.0.0)
-    redis (5.0.8)
-      redis-client (>= 0.17.0)
-    redis-client (0.18.0)
-      connection_pool
+    redis (4.8.1)
     redis-namespace (1.11.0)
       redis (>= 4)
     regexp_parser (2.8.2)
@@ -755,10 +752,10 @@ GEM
       sidekiq (>= 3.0)
     shoulda-matchers (5.3.0)
       activesupport (>= 5.2.0)
-    sidekiq (6.5.5)
-      connection_pool (>= 2.2.2)
+    sidekiq (6.5.12)
+      connection_pool (>= 2.2.5, < 3)
       rack (~> 2.0)
-      redis (>= 4.5.0)
+      redis (>= 4.5.0, < 5)
     signet (0.18.0)
       addressable (~> 2.8)
       faraday (>= 0.17.5, < 3.a)