CSS Library: add the USWDS utility breakpoint theme setting for medium-screen

[jamigibbs]

Description

The v3.9.0 release of USWDS includes the $theme-utility-breakpoints-custom setting which should be used to generate custom breakpoint utility classes. We can use this for the VA Design System's medium-screen breakpoint.

The medium-screen breakpoint is 768px.

The CSS Library theme settings should have something like the following added to it and all of the custom medium-screen Sass code removed.

@use "uswds-core" with (
  ...
  $theme-utility-breakpoints-custom: (
    "medium-screen": 768px
  )
);

Tasks

• Update the USWDS package in css-library to v3.9.0
• Update the CSS Library USWDS theme settings to use the $theme-utility-breakpoints-custom setting.
• Remove the custom medium-screen utility Sass code
• Generate the new utility classes

Acceptance Criteria

• custom utility breakpoint classes for medium-screen are generating and tested
• code has been cleaned up