Fix deprecations in opclasses initializer

The use of `table_exists?` in the opclasses support initializer triggers
a deprecation warning. Using `data_source_exists?` removes this
deprecation.

config/initializers/postgresql_opclasses_support.rb

@@ -81,7 +81,7 @@ module ActiveRecord
         if index_name.length > max_index_length
           raise ArgumentError, "Index name '#{index_name}' on table '#{table_name}' is too long; the limit is #{max_index_length} characters"
         end
-        if table_exists?(table_name) && index_name_exists?(table_name, index_name, false)
+        if data_source_exists?(table_name) && index_name_exists?(table_name, index_name, false)
           raise ArgumentError, "Index name '#{index_name}' on table '#{table_name}' already exists"
         end
         index_columns = quoted_columns_for_index(column_names, options).join(", ")