Monthly archive dropdown widget doesn't include custom classes.

[peterwilsoncc]

Description

Custom widget classes do not display on the front-end for archive dropdown widgets.

Step-by-step reproduction instructions

1. On WP Trunk, go to Appearance > Widgets
2. Add a monthly archive block
3. Format as a dropdown
4. Add some custom classes in the advanced section
5. Update the widgets and view on front end, the custom classes will be missing

In list format, IE as a ul, the custom classes is shown.

Expected behaviour

Custom classes apply to all layouts.

Actual behaviour

Custom classes do not apply to all layouts.

WordPress information

• WordPress version: Trunk @ WordPress/wordpress-develop@a3da548 | r51192
• Gutenberg version: Trunk @ 779bb17
• Are all plugins except Gutenberg deactivated? yes
• Are you using a default theme (e.g. Twenty Twenty-One)? Yes, Twenty Twenty-One

Device information

• Device: Laptop
• Operating system: macOS Big Sur
• Browser: Firefox Developer Edition 90.0b9

[thisissandip]

PR added: #32971