Update link for GitHub Copilot app modernization

[leereilly]

What's being changed (if available, include any code snippets, screenshots, or gifs):

The link to https://marketplace.visualstudio.com/items?itemName=ms-dotnettools.GitHubCopilotUpgradeAgent currently 404s. This PR replaces it with one that works.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[welcome]

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[Copilot]

Pull Request Overview

This PR fixes a broken link in the Copilot documentation by updating the URL to the correct GitHub Copilot app modernization extension in the Visual Studio Marketplace.

• Updates the broken marketplace link that was returning a 404 error
• Replaces the old extension ID ms-dotnettools.GitHubCopilotUpgradeAgent with the correct one ms-appmod.dotnet-modernization

[github-actions]

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
copilot/tutorials/upgrade-projects.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Sharra-writes]

@leereilly 👍

[github-actions]

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