docs(acr): update AcrImageSigner role

[lizMSFT]

The AcrImageSigner role is used for the ACR Content Trust feature. Since we plan to deprecate this feature, the role will also be deprecated in the future.

https://learn.microsoft.com/en-us/azure/container-registry/container-registry-content-trust-deprecation

[prmerger-automator]

@lizMSFT : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[lizMSFT]

@yizha1 update the description for acrimagesigner role

[learn-build-service-prod]

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[Copilot]

Pull Request Overview

This PR updates the documentation for the AcrImageSigner built-in role to reflect its planned deprecation. The change adds a deprecation notice indicating that the role will be removed on March 31, 2028, due to the deprecation of the ACR Content Trust feature.

Key changes:

• Updated AcrImageSigner role description to include deprecation notice and timeline
• Added reference link for migration guidance

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[ttorble]

@rolyon

Can you review the proposed changes?

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#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team