Workaround for devcontainer expired key issue

[westey-m]

Motivation and Context

Various Microsoft devcontainers including the dotnet one currently don't start due to an expired key issue.

Description

• Add a dockerfile to work around key issue
• Default to latest dotnet dev container
• Update feature versions

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 PR updates the .NET devcontainer setup to work around an upstream expired key issue and modernize the container configuration.

Changes:

• Introduces a custom .devcontainer/dotnet.Dockerfile that removes the Yarn APT repo with an expired GPG key to avoid apt-get update failures.
• Switches the devcontainer definition from a fixed base image tag to building from the new Dockerfile (effectively tracking the latest .NET dev container image).
• Bumps devcontainer feature versions for .NET, Azure CLI, and Docker-in-Docker.

Reviewed changes

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

File	Description
.devcontainer/dotnet/dotnet.Dockerfile	Adds a minimal Dockerfile that bases on the official .NET devcontainer image and removes the problematic Yarn APT source as a workaround.
.devcontainer/dotnet/devcontainer.json	Updates the devcontainer to build from the new Dockerfile, comments out the direct image reference, and refreshes feature versions.