Release v3.12.0 🚀

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Release GOV.UK Frontend v3.12.0, following our release process.

Done when:

This will be done when:

• the package has been published on npm
• the release has been created on GitHub
• the release has been communicated on Slack
• the sprint board has been updated

Then:

• Update the Design System to use Frontend v3.12.0
• Update the Prototype Kit to use Frontend v3.12.0
• Update the Frontend Docs to use GOV.UK Frontend v3.12.0