Move scheduler options to scheduler key in the sidekiq.yml

As requested by https://github.com/sidekiq-scheduler/sidekiq-scheduler/blob/v5.0.0.beta1/CHANGELOG.md#500beta1 and https://github.com/sidekiq-scheduler/sidekiq-scheduler/pull/412

```
[BREAKING CHANGE] Moves all sidekiq-scheduler config options under the scheduler key in the sidekiq.yml file #412

If you're migrating from v4 to v5, any sidekiq-scheduler config you may have in your sidekiq.yml should be moved under the scheduler key.
See #412 to see which are the config options that you need to move under the scheduler key.
```

config/sidekiq.yml

@@ -7,12 +7,13 @@
   - default
   - mailers
 
-:schedule:
-  HeartbeatJob:
-    every: ["5m", first_in: "0s"]
-  SubscriptionPlacementJob:
-    every: "5m"
-  SubscriptionConfirmJob:
-    every: "5m"
-  OrderCycleClosingJob:
-    every: "5m"
+:scheduler:
+  :schedule:
+    HeartbeatJob:
+      every: ["5m", first_in: "0s"]
+    SubscriptionPlacementJob:
+      every: "5m"
+    SubscriptionConfirmJob:
+      every: "5m"
+    OrderCycleClosingJob:
+      every: "5m"