fix(module:cascader): sync class name with ant-design

[rppig42]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

when Cascader search bar is not empty and focused, the font color is pure black

Issue Number: #6020

What is the new behavior?

when Cascader search bar is not empty and focused, the font color is gray

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Due to the lack of experience of git collaboration, I ruined the old pull request #6021. I've opened another pr, @hsuanxyz pls take a look.

This pr synced the class name to be consistent with ant design so that the styles from ant-design would take effect.

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #6030 (65d8492) into master (22aea7e) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6030      +/-   ##
==========================================
+ Coverage   89.91%   89.95%   +0.04%     
==========================================
  Files         465      465              
  Lines       14042    14042              
  Branches     2307     2307              
==========================================
+ Hits        12626    12632       +6     
+ Misses        869      863       -6     
  Partials      547      547              

Impacted Files	Coverage Δ
components/cascader/cascader.component.ts	94.67% <ø> (ø)
components/tabs/tab-nav-bar.component.ts	81.62% <0.00%> (+0.70%)	⬆️
components/table/src/table-data.service.ts	84.61% <0.00%> (+6.15%)	⬆️

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[hsuanxyz]

LGTM