fix: ⬆️ BackTop not working in iframe

[afc163]

🤔 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

💡 Background and solution

• 在 iframe 里不生效的问题。
• 优化了 BackTop 的演示文档。
• 提升了滚动性能。

📝 Changelog

Language	Changelog
🇺🇸 English	Fix BackTop not working in iframe and improve it's perfermance.
🇨🇳 Chinese	修复 BackTop 在 iframe 里不生效并提升了组件性能。

☑️ Self Check before Merge

⚠️ Please check all items below before review. ⚠️

• 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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View rendered components/back-top/demo/basic.md
View rendered components/back-top/demo/custom.md

[tests-checker]

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

[ant-design-bot]

[codesandbox-ci]

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This pull request is automatically built and testable in CodeSandbox.

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[codecov]

Codecov Report

Merging #22788 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #22788      +/-   ##
==========================================
+ Coverage   98.28%   98.31%   +0.03%     
==========================================
  Files         361      361              
  Lines        7211     7235      +24     
  Branches     1933     1985      +52     
==========================================
+ Hits         7087     7113      +26     
+ Misses        124      122       -2

Impacted Files	Coverage Δ
components/_util/getScroll.tsx	100% <100%> (+20%)	⬆️
components/_util/scrollTo.ts	100% <100%> (ø)	⬆️
components/back-top/index.tsx	100% <100%> (ø)	⬆️

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[afc163]

我补一下覆盖率。