feat(module:timeline): add gray as a default color

[wzhudev]

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) Not needed
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] 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

It works with the latest style update #3921.

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[codecov]

Codecov Report

Merging #3922 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3922   +/-   ##
=======================================
  Coverage   95.29%   95.29%           
=======================================
  Files         777      777           
  Lines       14825    14825           
  Branches     1910     1910           
=======================================
  Hits        14128    14128           
  Misses        260      260           
  Partials      437      437

Impacted Files	Coverage Δ
components/core/testing/public-api.ts	100% <ø> (ø)	⬆️
components/timeline/nz-timeline-item.component.ts	100% <100%> (ø)	⬆️
components/tabs/nz-tabset.component.ts	93.19% <0%> (ø)	⬆️

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[hsuanxyz]

看上去多了一些奇怪的文件

[wzhudev]

看上去多了一些奇怪的文件

Oh, fuck vim!

[darshanrampatel]

@wendzhue @hsuanxyz Looks like you have used both "grey" and "gray" - only one of these should be used.

[wzhudev]

@wendzhue @hsuanxyz Looks like you have used both "grey" and "gray" - only one of these should be used.

Hi! Thank you for pointing that out. We have changed all grey to gray.

[vthinkxie]

test failed

[vthinkxie]

LGTM