Subtle content switcher 🌳

[laurenmrice]

Acceptance criteria

• This enhancement provides a secondary visual style for the content switcher with lower contrast but still retains enough contrast to meet WCAG requirements and provide the user with an easily identifiable ‘selected’ state.
• We need to create new design tokens for this enhancement, create a final design spec for implementation, and update our documentation in code, the website, and the figma kit.
• We also need to update the corner radius in the high contrast content switcher across the board.

Ref related issue: #7509

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### Tasks
- [ ] https://github.com/carbon-design-system/carbon/issues/17328
- [ ] https://github.com/carbon-design-system/carbon/issues/17329
- [ ] https://github.com/carbon-design-system/carbon/issues/17332
- [ ] https://github.com/carbon-design-system/carbon/issues/17331
- [ ] https://github.com/carbon-design-system/carbon/issues/17330

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