Revert devcontainer bug workaround

[westey-m]

Motivation and Context

There was a bug in the public devcontainer for a while, which required a workaround, but is resolved now.

Description

• Reverting workaround

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This pull request reverts a temporary workaround that was needed for a bug in the devcontainer base image. The workaround addressed an expired Yarn GPG key issue (tracked in devcontainers/images#1752) that caused apt-get update failures. Now that the upstream issue is resolved, the repository can return to using the official devcontainer image directly.

Changes:

• Removes the custom Dockerfile workaround (dotnet.Dockerfile) that was used to delete the problematic Yarn repository
• Restores direct usage of the mcr.microsoft.com/devcontainers/dotnet image in the devcontainer configuration

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.devcontainer/dotnet/dotnet.Dockerfile	Deleted custom Dockerfile that was only needed to work around the Yarn GPG key issue
.devcontainer/dotnet/devcontainer.json	Reverted from custom build to direct image usage, consistent with the Python devcontainer pattern