🐛 Fix disabled and selected date cell style of DatePicker

[afc163]

🤔 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?

close #15580

💡 Solution

Fixed style to:

📝 Changelog description

Describe changes from userside, and list all potential break changes or other risks.

1. English description

• 🐛 Fix disabled and selected date cell style of DatePicker. 默认选中了被禁用的日期时的选中样式优化 #15580

2. Chinese description (optional)

☑️ 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 #15608 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15608      +/-   ##
==========================================
+ Coverage   94.41%   94.49%   +0.07%     
==========================================
  Files         250      250              
  Lines        6646     6646              
  Branches     1942     1909      -33     
==========================================
+ Hits         6275     6280       +5     
+ Misses        370      365       -5     
  Partials        1        1

Impacted Files	Coverage Δ
components/_util/wave.tsx	87.5% <0%> (+2.88%)	⬆️
components/back-top/index.tsx	91.93% <0%> (+3.22%)	⬆️

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