Storybook: Set DateCalendar and DateRangeCalendar components as private.

[juanfra]

What?

I was trying out a few components on a separate project, and I didn't realized DateCalendar and DateRangeCalendar were private as it wasn't highlighted in storybook.

Testing Instructions

1. Run nvm use && npm run storybook:dev
2. Open the storybook locally.
3. Check the DateCalendar and DateRangeCalendar stories.

Screenshots or screencast

[github-actions]

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Co-authored-by: juanfra <[email protected]>
Co-authored-by: Mamaduka <[email protected]>

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[github-actions]

Flaky tests detected in ebefd1b.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/17384010713
📝 Reported issues:

• [Flaky Test] Redirection after template creation #60678 in /test/e2e/specs/site-editor/site-editor-url-navigation.spec.js

[Mamaduka]

Thanks, @juanfra!