Update Vue School summer banner

[nicodevs]

This PR updates the banner introduced by #1407

The banner at the top of https://pinia.vuejs.org/ informs users about Vue School’s Summer Sale promotion.

The banner is closable and opens Vue School's Summer Sale promo page in a new tab. The banner will not reappear if a user has closed it (stored in localStorage).

Please note that this banner includes a countdown. The component uses @chenfengyuan/vue-countdown.

Click here to see screenshots of the banner

[netlify]

✅ Deploy Preview for pinia-official ready!

Name	Link
🔨 Latest commit	746d59f
🔍 Latest deploy log	https://app.netlify.com/sites/pinia-official/deploys/62deb2967d4d830008b541d1
😎 Deploy Preview	https://deploy-preview-1457--pinia-official.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site settings.

[codecov-commenter]

Codecov Report

Merging #1457 (1102bf8) into v2 (6be93f2) will not change coverage.
The diff coverage is n/a.

❗ Current head 1102bf8 differs from pull request most recent head 746d59f. Consider uploading reports for the commit 746d59f to get more accurate results

@@           Coverage Diff           @@
##               v2    #1457   +/-   ##
=======================================
  Coverage   97.59%   97.59%           
=======================================
  Files           9        9           
  Lines         416      416           
  Branches      109      109           
=======================================
  Hits          406      406           
  Misses          5        5           
  Partials        5        5           

Impacted Files	Coverage Δ
packages/pinia/src/types.ts	100.00% <ø> (ø)
packages/testing/src/testing.ts	92.59% <ø> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us.