fix(module:drawer): fix z-index level

[hsuanxyz]

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?

Issue Number: #3402

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[codecov]

Codecov Report

Merging #3405 into master will decrease coverage by 1.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3405      +/-   ##
==========================================
- Coverage   96.66%   95.48%   -1.18%     
==========================================
  Files         609      609              
  Lines       13187    13187              
  Branches     1803     1803              
==========================================
- Hits        12747    12592     -155     
- Misses        199      215      +16     
- Partials      241      380     +139

Impacted Files	Coverage Δ
components/core/util/getMentions.ts	50% <0%> (-16.67%)	⬇️
components/core/scroll/nz-scroll.service.ts	66.03% <0%> (-13.21%)	⬇️
components/core/polyfill/request-animation.ts	20% <0%> (-12%)	⬇️
components/slider/nz-slider-step.component.ts	88.88% <0%> (-11.12%)	⬇️
components/i18n/nz-i18n.service.ts	80.95% <0%> (-9.53%)	⬇️
components/core/util/textarea-caret-position.ts	42.1% <0%> (-8.78%)	⬇️
components/breadcrumb/nz-breadcrumb.component.ts	56.52% <0%> (-6.53%)	⬇️
components/slider/nz-slider-marks.component.ts	90.47% <0%> (-4.77%)	⬇️
components/select/nz-option-container.component.ts	95.34% <0%> (-4.66%)	⬇️
components/core/util/throttleByAnimationFrame.ts	90.9% <0%> (-4.55%)	⬇️
... and 54 more

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