Update logic for overriding :algorithm in add_index

Rails (at least version 7.1) raises an error if the `:algorithm` option is set to :default, before this code gets
invoked.

Instead of allowing a custom `:default` option, we can instead just check whether the options[:algorithm] key is
present. This is what the `remove_index` method below does, and allows us to use `algorithm: nil` to disable the
`:concurrently` option.

Author: jordan-brough

lib/safe-pg-migrations/plugins/statement_insurer.rb

@@ -65,11 +65,7 @@ module StatementInsurer # rubocop:disable Metrics/ModuleLength
     end
 
     def add_index(table_name, column_name, **options)
-      if options[:algorithm] == :default
-        options.delete :algorithm
-      else
-        options[:algorithm] = :concurrently
-      end
+      options[:algorithm] = :concurrently unless options.key?(:algorithm)
 
       SafePgMigrations.say_method_call(:add_index, table_name, column_name, **options)