feat: add back @border-radius-sm

[akalittle]

🤔 This is a ...

• [x ] Other ( add less var)

🔗 Related issue link

close #20817

💡 Background and solution

1. Add a extra less var to make it compatible from migrating V3 to V4.

📝 Changelog

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🇺🇸 English
🇨🇳 Chinese

☑️ Self Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

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[codecov]

Codecov Report

Merging #20818 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #20818   +/-   ##
=======================================
  Coverage   97.47%   97.47%           
=======================================
  Files         296      296           
  Lines        6880     6880           
  Branches     1911     1912    +1     
=======================================
  Hits         6706     6706           
  Misses        174      174

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