refactor(module:pageheader): insure compatibility RxJs 6

[Nicoss54]

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?

• 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?

paheHeader use observed getter insteadr observers which is not compatible with RxJs 6 still suppoprt by Angular core

Issue Number: N/A

What is the new behavior?

pagheHeader usethe oberservers property

Does this PR introduce a breaking change?

• Yes
• No

Other information

[zorro-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.94%. Comparing base (f0e9d7f) to head (e2588a0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9052   +/-   ##
=======================================
  Coverage   91.94%   91.94%           
=======================================
  Files         559      559           
  Lines       19773    19773           
  Branches     3050     3050           
=======================================
  Hits        18181    18181           
  Misses       1267     1267           
  Partials      325      325           

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[Laffery]

Same to #8454.

This 'observed' property does not yet exist in rxjs6. Since angular/core still supports rxjs6, we can't do this yet.

Further more, Line 134 should use observers instead of observed as well

[Nicoss54]

Same to #8454.

This 'observed' property does not yet exist in rxjs6. Since angular/core still supports rxjs6, we can't do this yet.

Further more, Line 134 should use observers instead of observed as well

done

[Laffery]

LGTM