[Aspire extension] Phase 7: Migrate legacy built-in Aspire projects

[vhvb1989]

Parent epic: #9479

Depends on the Aspire extension becoming the default for new projects.

Objective

Move existing host: containerapp plus language: dotnet Aspire projects onto the extension without breaking old projects or silently rewriting configuration.

Scope

• Continue recognizing the legacy marker as Aspire only when the referenced project is an AppHost.

• Prefer the extension for legacy projects once the installed version meets compatibility requirements.

• Retain built-in fallback during the migration window.

• Never switch implementations after an operation has started.

• Add an explicit migration command, for example:

  azd aspire migrate

• Update azure.yaml to host: aspire, remove the obsolete language marker, configure the Aspire provisioning provider, and add extension requirements where needed.

• Provide preview/confirmation and preserve comments and unrelated configuration where possible.

• Add regression coverage for old templates, generated projects, CI pipelines, App Service, and Container Apps.

• Publish migration and rollback guidance.

Exit criteria

• Legacy projects run successfully through the extension without mandatory edits.
• Explicit migration produces the new canonical configuration.
• Built-in fallback usage and extension migration adoption can be measured safely.