fix(module:select): fast scroll causes the scroll to bottom to misbehave

[ParsaArvanehPA]

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?

Fast scrolling causes CDK's scrolledIndexChange to not report some one the indexes(most probably they use a debouneTime on their observable).
By checking the relative index instead of the absolute index the issue can be fixed.

Issue Number: #8752

What is the new behavior?

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 93.15%. Comparing base (dd42ad9) to head (e91b734).

Additional details and impacted files

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  Coverage   93.15%   93.15%           
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  Files         538      538           
  Lines       18457    18457           
  Branches     2816     2816           
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  Hits        17193    17193           
  Misses       1226     1226           
  Partials       38       38           

Flag	Coverage Δ
	93.15% <100.00%> (?)

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