Change Redis#exists calls to Redis#exists? to avoid deprecation warning (#14191)

lib/redis/namespace_extensions.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+class Redis
+  module NamespaceExtensions
+    def exists?(*args, &block)
+      call_with_namespace('exists?', *args, &block)
+    end
+  end
+end
+
+Redis::Namespace::COMMANDS['exists?'] = [:first]
+Redis::Namespace.prepend(Redis::NamespaceExtensions)