docs: remove dead deno.land/std link from README (registry retired)

[sanjibani]

Summary

The README's IMPORTANT notice references https://deno.land/std, which now returns HTTP 404. The deno.land/x registry has been retired and no longer serves packages.

Changes

Updated the IMPORTANT notice to drop the dead link and the now-false 'older versions up till 0.224.0 are still available at deno.land/std' claim, while keeping the working https://jsr.io/@std link.

 > [!IMPORTANT]
-> Newer versions of the Standard Library are now hosted on
-> [JSR](https://jsr.io/@std). Older versions up till 0.224.0 are still available
-> at [deno.land/std](https://deno.land/std).
+> The Standard Library is now hosted on [JSR](https://jsr.io/@std). The legacy
+> deno.land/std registry has been retired and no longer serves any packages.

Verification

• https://deno.land/std returns 404 (curl HEAD + GET)
• https://jsr.io/@std returns 200 (still the canonical home)

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (cdf74a8) to head (3802826).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7187      +/-   ##
==========================================
- Coverage   94.57%   94.57%   -0.01%     
==========================================
  Files         636      637       +1     
  Lines       52142    52150       +8     
  Branches     9401     9401              
==========================================
+ Hits        49315    49319       +4     
- Misses       2249     2254       +5     
+ Partials      578      577       -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[bartlomieju]

The README's IMPORTANT notice references https://deno.land/std, which now returns HTTP 404.

No, it doesn't return 404.

The deno.land/x registry has been retired and no longer serves packages.

Serves packages just fine. Where are you getting this information?