fix(module:date-picker): fix z-index to auto in inline mode

[cipchk]

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: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[zorro-bot]

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[codecov]

Codecov Report

Merging #7172 (8d1b1d2) into master (77db914) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7172      +/-   ##
==========================================
- Coverage   91.67%   91.66%   -0.01%     
==========================================
  Files         486      486              
  Lines       15868    15868              
  Branches     2587     2587              
==========================================
- Hits        14547    14546       -1     
- Misses       1007     1008       +1     
  Partials      314      314              

Impacted Files	Coverage Δ
components/core/util/text-measure.ts	92.10% <0.00%> (-0.88%)	⬇️

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