fix(Steps): add subTitle propType declaration for Steps.Step component

[jtsang4]

🤔 This is a ...

• TypeScript definition update

🔗 Related issue link

There is no link, just a issue find in usage that the <Steps.Step> component dosen't have subTitle prop type declaration but it is supported by rc-steps and exposed by the document.

💡 Background and solution

Add subTitle prop in StepProps interface.

📝 Changelog

Language	Changelog
🇺🇸 English	Fix <Steps.Step> component's subTitle prop types
🇨🇳 Chinese	修复 <Steps.Step> 组件 subTitle 属性类型

☑️ 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 #18525 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #18525   +/-   ##
=======================================
  Coverage   96.73%   96.73%           
=======================================
  Files         280      280           
  Lines        7532     7532           
  Branches     2095     2095           
=======================================
  Hits         7286     7286           
  Misses        244      244           
  Partials        2        2

Impacted Files	Coverage Δ
components/steps/index.tsx	100% <ø> (ø)	⬆️

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