fix: Adding ant-upload-btn class to the upload component

[PixelDust22]

This is a ...

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

What's the background?

It was discovered that when the screen is wider than the upload component content, clicking outside of the content bounding box does not trigger the upload dialog. Dragging a file outside of that box causes the browser to open that file instead of uploading the file.

This bug does not exist on the React version of ant design. After comparing the two versions, it was discovered that the Vue implementation is missing a class on the wrapping span, causing the layout for that wrapper to be broken.

This PR addresses the issue by adding back the missing class name.

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 #1742 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1742   +/-   ##
=======================================
  Coverage   88.09%   88.09%           
=======================================
  Files         157      157           
  Lines        5416     5416           
  Branches     1524     1524           
=======================================
  Hits         4771     4771           
  Misses        576      576           
  Partials       69       69

Impacted Files	Coverage Δ
components/upload/Upload.jsx	84.15% <ø> (ø)	⬆️

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[tangjinzhou]

you should change this line to <VcUpload {...vcUploadProps} class={${prefixCls}-btn}>
https://github.com/vueComponent/ant-design-vue/blob/master/components/upload/Upload.jsx#L276

[PixelDust22]

The requested changes have been made.

[github-actions]

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