Prepare 7.0 breaking changes

[aduth]

This is a staging area for breaking changes we'd like to implement in a future major version release.

Currently, it includes:

Breaking Changes

• Sass artifacts now require Dart Sass. The Ruby Sass and LibSass (C) compilers are no longer supported.
• To improve performance of the default CSS bundle, responsive utility breakpoints other than tablet: have been disabled by default. Previously, mobile-lg: and desktop: utilities were also enabled. You can restore the original settings by overriding the $theme-utility-breakpoints Sass variable.
• Image URLs in stylesheets are no longer processed by asset-path-if-exists. For Rails integrations, consider using the $theme-image-path Sass variable in combination with Sprockets resolve_assets_in_css_urls option.
• The deprecated $image-path Sass variable setting has been removed. Use $theme-image-path instead.
• The deprecated alerts/success-badge.svg image has been removed. Use alerts/success.svg instead.

[aduth]

Other ideas:

• Remove components for improved upstream compatibility with USWDS and reduced global style pollution:
  • Badges
    • Could be implemented specific to IdP
    • Unsure of long-term desire to keep
  • Cards
    • Conflicting name and implementation between this and USWDS component of the same name
  • Spinner
    • It's a bad practice for us to be recommending
    • Could be implemented specific to IdP
    • Consider consolidating / reconciling with IdP "Spinner Button"
  • Code formatting
    • Pollutes global styles
    • Not applicable to every project
• Remove project-specific or documentation-specific images

[aduth]

It probably makes sense to release the next major version in tandem with USWDS v3, which is slated for release next month.

[aduth]

Other ideas:

I still like many of these ideas, but we can save them for a future major release.

[aduth]

@zachmargolis I think I'd like to include these changes in the upcoming release. Could I tap you for a review?

[zachmargolis]

LGTM

[aduth]

Visual regression failure is from the removal of the "Limiting Alert Width" documentation section, which is an expected breaking change.