CI: remove manual PowerShell install from test-proxy job (e2e-tests.yml)

[priya-kinthali]

Description:
This PR changes the test-proxy job to use mcr.microsoft.com/devcontainers/dotnet:10.0 container image instead of ubuntu:22.04 and removes the manual PowerShell installation step that used wget/dpkg, which was unsafe due to potential malicious .deb file substitution.

Related issue:
Add link to the related issue.

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[Copilot]

Pull request overview

This PR enhances the security of the test-proxy job in the e2e tests workflow by replacing the ubuntu:22.04 base container image with mcr.microsoft.com/devcontainers/dotnet:10.0, which comes with PowerShell pre-installed. This change eliminates the need for a manual PowerShell installation step that previously downloaded and installed packages using wget and dpkg, which posed a security risk due to potential malicious package substitution.

Changes:

• Switched container image from ubuntu:22.04 to mcr.microsoft.com/devcontainers/dotnet:10.0
• Removed the manual PowerShell installation step that used insecure wget/dpkg commands

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