Storybook: update background color for the use with caution label

[jamigibbs]

Chromatic

https://storybook-maturity-label--65a6e2ed2314f7b8f98609d8.chromatic.com

Description

This fixes an issue where we lost the maturity color for the warning label. The colors were selected based on the maturity scale here: https://design.va.gov/about/maturity-scale

Screenshots

before

after

QA Checklist

• Component maintains 1:1 parity with design mocks
• Text is consistent with what's been provided in the mocks
• Component behaves as expected across breakpoints
• Accessibility expert has signed off on code changes (if applicable. If not applicable provide reason why)
• Designer has signed off on changes (if applicable. If not applicable provide reason why)
• Tab order and focus state work as expected
• Changes have been tested against screen readers (if applicable. If not applicable provide reason why)
• New components are covered by e2e tests; updates to existing components are covered by existing test suite
• Changes have been tested in vets-website using Verdaccio (if applicable. If not applicable provide reason why)

Acceptance criteria

• QA checklist has been completed
• Screenshots have been attached that cover desktop and mobile screens

Definition of done

• Documentation has been updated, if applicable
• A link has been provided to the originating GitHub issue