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 all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ bartlomieju
❌ sanjibani
_{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 94.64%. Comparing base (f0431c2) to head (7321e20).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7187      +/-   ##
==========================================
- Coverage   94.64%   94.64%   -0.01%     
==========================================
  Files         629      629              
  Lines       51895    51643     -252     
  Branches     9373     9323      -50     
==========================================
- Hits        49116    48877     -239     
+ Misses       2211     2202       -9     
+ Partials      568      564       -4     

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[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?