feat(date): fix focus loss when using navbar with keyboard

[damienrobson-sage]

The Navbar of the internal date picker component has been memoized to ensure that it doesn't rerender unless it absolutely needs to (which should be never as it's just two buttons; the month name isn't part of the navbar, it's just styled that way)

fix #7158

Proposed behaviour

Prevent the month navigation buttons from losing focus when activated via keyboard

Current behaviour

The buttons of the navigation bar in the Date Picker lose focus when activated using keyboard controls

Checklist

• Commits follow our style guide
• Related issues linked in commit messages if required
• Screenshots are included in the PR if useful
• All themes are supported if required
• Unit tests added or updated if required
• Playwright automation tests added or updated if required
• Storybook added or updated if required
• Translations added or updated (including creating or amending translation keys table in storybook) if required
• Typescript d.ts file added or updated if required
• Related docs have been updated if required

QA

• Tested in provided StackBlitz sandbox/Storybook
• Add new Playwright test coverage if required
• Carbon implementation matches Design System/designs
• UI Tests GitHub check reviewed if required

Testing instructions

Use the Storybook examples to ensure that the buttons don't lose focus when navigating between months using the keyboard