Silence deprecation warnings in secrets

We were getting a ton of deprecation warnings:

```
DEPRECATION WARNING: `Rails.application.secrets` is deprecated in favor
of `Rails.application.credentials` and will be removed in Rails 7.2
```

However, we don't plan to replace the secrets with credentials in the
foreseeable future because it would affect existing Consul Democracy
installations, so for now we're simply silencing the warnings.

config/application.rb

@@ -21,6 +21,14 @@ Bundler.require(*Rails.groups)
 
 module Consul
   class Application < Rails::Application
+    def secrets
+      Rails.deprecator.silence { super }
+    end
+
+    def secret_key_base
+      Rails.deprecator.silence { super }
+    end
+
     config.load_defaults 7.0
 
     # Keep belongs_to fields optional by default, because that's the way