docs(common): update reference url to ICU docs

[superole]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/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
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Documentation for the i18n-plural-pipe links to an old location of the ICU User Guide. It is still active, but the page clearly states that:

The ICU User Guide has moved to https://unicode-org.github.io/icu/userguide/.
Please update your bookmarks.
This obsolete site will be deleted.

Issue Number: N/A

What is the new behavior?

Documentation for the i18n-plural-pipe links to the current location of the ICU User Guide.

Does this PR introduce a breaking change?

• Yes
• No

Other information

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[AndrewKushnir]

@superole thanks for improving Angular docs 👍

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[AndrewKushnir]

Merge-assistance: docs-only change, does not require g3 presubmit. The PR is ready for merge.

[AndrewKushnir]

This PR was merged into the repository by commit 26ad19e.

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