Fix: keep className given to expandIcon in Collapse component

[gpetrioli]

🤔 This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

🔗 Related issue link

fixes #19158

💡 Background and solution

use classNames method to add the `${prefixCls}-arrow` to the existing classNames instead of overwriting them.

📝 Changelog

Class name given to the expandIcon of the Collapse component will be maintained so that might pose a change if users expected it to be lost.

Language	Changelog
🇺🇸 English	Keep className given to expandIcon in Collapse component
🇨🇳 Chinese	保留 Collapse 组件中 expandIcon 自定义 className

☑️ 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

[tests-checker]

Could you please add tests to make sure this change works as expected?

[netlify]

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[shaodahong]

CI broken

[codecov]

Codecov Report

Merging #19160 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #19160   +/-   ##
=======================================
  Coverage   97.32%   97.32%           
=======================================
  Files         282      282           
  Lines        7548     7548           
  Branches     2110     2110           
=======================================
  Hits         7346     7346           
  Misses        201      201           
  Partials        1        1

Impacted Files	Coverage Δ
components/collapse/Collapse.tsx	95% <100%> (ø)	⬆️

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[shaodahong]

Thanks