Prepare database_cleaner-mongoid for ruby 3

- This came up when I was trying to squash some deprecation warnings

DatabaseCleaner/database_cleaner-mongoid#16

Author: tobyprivett

spec/support/database_cleaner.rb

@@ -6,7 +6,7 @@
 RSpec.configure do |config|
   # Clean the registrations and users databases before running tests
   config.before(:suite) do
-    DatabaseCleaner[:mongoid].strategy = :deletion
+    DatabaseCleaner[:mongoid].strategy = [:deletion]
     DatabaseCleaner[:mongoid, { db: :users }].strategy = :deletion
 
     DatabaseCleaner.clean