Replace `default` overrides from `load_schema!` to `attribute`

[krororo]

This PR fixes overwriting with load_schema! will not work from Rails 7.2. see: https://github.com/rails/rails/commit/e0a55b038f7f2f50d1467876558be183be6cedaa

The load_schema! method is now called inside the _default_attributes method due to the https://github.com/rails/rails/commit/e0a55b038f7f2f50d1467876558be183be6cedaa change. https://github.com/rails/rails/blob/v7.2.0.beta3/activerecord/lib/active_record/attributes.rb#L240-L252

The backtrace at that time is below.

=>#0	ActiveRecord::Bitemporal::Bitemporalize::ClassMethods#load_schema! at ~/github/activerecord-bitemporal/lib/activerecord-bitemporal.rb:69
  #1	block in load_schema at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activerecord-7.2.0.beta3/lib/active_record/model_schema.rb:539
  #2	[C] Monitor#synchronize at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activerecord-7.2.0.beta3/lib/active_record/model_schema.rb:536
  #3	ActiveRecord::ModelSchema::ClassMethods#load_schema at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activerecord-7.2.0.beta3/lib/active_record/model_schema.rb:536
  #4	ActiveRecord::ModelSchema::ClassMethods#columns_hash at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activerecord-7.2.0.beta3/lib/active_record/model_schema.rb:429
  #5	block {|connection=#<ActiveRecord::ConnectionAdapters::Postg...|} in _default_attributes at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activerecord-7.2.0.beta3/lib/active_record/attributes.rb:243
  #6	ActiveRecord::ConnectionAdapters::ConnectionPool#with_connection(prevent_permanent_checkout=false) at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activerecord-7.2.0.beta3/lib/active_record/connection_adapters/abstract/connection_pool.rb:407
  #7	ActiveRecord::ConnectionHandling#with_connection(prevent_permanent_checkout=false, block=#<Proc:0x00007c5ceb31b680 /home/...) at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activerecord-7.2.0.beta3/lib/active_record/connection_handling.rb:296
  #8	ActiveRecord::Attributes::ClassMethods#_default_attributes at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activerecord-7.2.0.beta3/lib/active_record/attributes.rb:242
  #9	ActiveModel::AttributeRegistration::ClassMethods#attribute_types at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activemodel-7.2.0.beta3/lib/active_model/attribute_registration.rb:38
  #10	ActiveRecord::AttributeMethods::ClassMethods#_has_attribute?(attr_name="type") at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activerecord-7.2.0.beta3/lib/active_record/attribute_methods.rb:260
  #11	ActiveRecord::Inheritance::ClassMethods#new(attributes={:name=>"Mami"}, block=nil) at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activerecord-7.2.0.beta3/lib/active_record/inheritance.rb:61
  #12	ActiveRecord::Persistence::ClassMethods#create!(attributes={:name=>"Mami"}, block=nil) at ~/installs/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/activerecord-7.2.0.beta3/lib/active_record/persistence.rb:54

Therefore, default set inside _default_attributes were no longer reflected in @default_attributes. Because the new ActiveModel::AttributeSet object will be set to @default_attributes.

The load_schema! method was overridden to handle precision correctly. (#57) However, this behavior has been fixed to be handled by unspecifying the type in the attribute method. (https://github.com/rails/rails/pull/39830)