chore: add cardinality aggregation type for search query

[AaronZyLee]

Description of changes:

Add cardinality aggregation type in open search doc.

This doc responds to PR https://github.com/aws-amplify/amplify-category-api/pull/2127

Official doc: https://www.elastic.co/guide/en/elasticsearch/reference/7.10/search-aggregations-metrics-cardinality-aggregation.html

This is not released in the CLI yet. Will hold the merge until the CLI is released with latest API category.

Related GitHub issue #, if available:

Instructions

If this PR should not be merged upon approval for any reason, please submit as a DRAFT

Which product(s) are affected by this PR (if applicable)?

• [x] amplify-cli
• [ ] amplify-ui
• [ ] amplify-studio
• [ ] amplify-hosting
• [ ] amplify-libraries

Which platform(s) are affected by this PR (if applicable)?

• [ ] JS
• [ ] Swift
• [ ] Android
• [ ] Flutter
• [ ] React Native

Please add the product(s)/platform(s) affected to the PR title

Checks

• [x] Does this PR conform to the styleguide?

• [ ] Does this PR include filetypes other than markdown or images? Please add or update unit tests accordingly.

• [ ] Are any files being deleted with this PR? If so, have the needed redirects been created?

• [ ] Are all links in MDX files using the MDX link syntax rather than HTML link syntax?
  ref: MDX: [link](https://docs.amplify.aws/) HTML: <a href="https://docs.amplify.aws/">link</a>

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• [ ] Ready to merge

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