Add App Center retirement warnings

[balcsida]

Visual Studio App Center is scheduled for retirement on March 31, 2025 - the docs should include a warning about this.

[prmerger-automator[bot]]

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

[Court72]

@vijayma

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

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

[juliakm]

#sign-off

[prmerger-automator[bot]]

Invalid command: '#sign-off'. Only the assigned author of one or more file in this PR can sign off. @vijayma

[juliakm]

#sign-off

[prmerger-automator[bot]]

Invalid command: '#sign-off'. Only the assigned author of one or more file in this PR can sign off. @vijayma

[juliakm]

#sign-off

[prmerger-automator[bot]]

Invalid command: '#sign-off'. Only the assigned author of one or more file in this PR can sign off. @vijayma