[EngSys] update script to check for external dependency upgrades

[jeremymeng]

This PR re-enables reporting of available major upgrades for our external dependencies using pnpm outdated command.

[Copilot]

Pull Request Overview

This PR re-enables the external dependency upgrade monitoring system by switching from pnpm update --latest to pnpm outdated --format json --recursive for better detection of available major version upgrades. The changes improve the accuracy and reliability of dependency upgrade reporting.

Key changes:

• Replaces the previous approach of running updates and parsing git diff output with a more direct method using pnpm's outdated command
• Re-enables GitHub issue creation for dependency upgrades that was previously commented out
• Adds proper JSON parsing and filtering to exclude Azure-internal packages from upgrade reporting

Comments suppressed due to low confidence (1)

eng/scripts/check-external-dependency.ps1:11

• The deprecated dependency regex variable is removed but may still be referenced elsewhere in the codebase. Consider verifying that this variable is not used in other parts of the script or related files.

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