docs: point Quick Start zencdn links at hosted 8.23.6

[Hashim1999164]

Description

The README Quick Start still points vjs.zencdn.net at 8.23.9. That path currently returns 403, so the first CDN example does not load CSS/JS.

Verified today:

• 8.23.6 is available on vjs.zencdn.net (200)
• 8.23.7 through 8.23.9 return 403

This updates only the Fastly/zencdn Quick Start tags to 8.23.6. The unpkg and cdnjs examples stay on 8.23.9, which those hosts do serve.

Specific Changes proposed

• README Quick Start: vjs.zencdn.net/8.23.9 -> vjs.zencdn.net/8.23.6 for CSS and JS

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
  • Has no DOM changes which impact accessiblilty or trigger warnings (e.g. Chrome issues tab)
  • Has no changes to JSDoc which cause npm run docs:api to error
• Reviewed by Two Core Contributors

Fixes #9051

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.35%. Comparing base (bf013ba) to head (7b51d06).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9215   +/-   ##
=======================================
  Coverage   84.35%   84.35%           
=======================================
  Files         120      120           
  Lines        8177     8177           
  Branches     1975     1975           
=======================================
  Hits         6898     6898           
  Misses       1279     1279           

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