Remove stale Aspire 9.x references from getting started docs

[davidfowl]

Summary

• remove stale Aspire 9.x framing from the affected get-started docs
• keep the guidance current and product-centric instead of release-history-centric
• preserve the latest origin/main wording where it already covered the same scenarios

Closes #576.

What changed

• updated csharp-service-defaults.mdx to describe current template behavior without the old Aspire 9.2 callout
• updated the C# and TypeScript existing-app guides to remove in Aspire 9 from the deprecated AddNpmApp note
• rebased onto origin/main, which already included equivalent wording refreshes for the dev-containers and github-codespaces pages

Validation

• verified there are no remaining Aspire 9.x references under src/frontend/src/content/docs/get-started
• doc-tested the affected rendered pages locally on the docs site and confirmed the stale wording is gone
• pnpm --dir ./src/frontend run build reached static-route generation, then hit the same environment-specific hang seen earlier in this workspace

[Copilot]

Pull request overview

Removes outdated Aspire 9.x version framing from getting-started documentation to keep guidance current and product-centric, aligning with the intent of issue #576.

Changes:

• Reworded the C# Service Defaults note to describe current template behavior without the “Starting with Aspire 9.2” callout.
• Removed the “in Aspire 9” reference from the deprecated AddNpmApp note in both C# and TypeScript existing-app guides.
• Confirmed no remaining “Aspire 9” references under src/frontend/src/content/docs/get-started.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/frontend/src/content/docs/get-started/csharp-service-defaults.mdx	Rephrases the template behavior note to be version-neutral while preserving the intent (health check tracing exclusion).
src/frontend/src/content/docs/get-started/add-aspire-existing-app-typescript-apphost.mdx	Updates the AddNpmApp deprecation note to remove the Aspire 9 version reference.
src/frontend/src/content/docs/get-started/add-aspire-existing-app-csharp-apphost.mdx	Updates the AddNpmApp deprecation note to remove the Aspire 9 version reference.

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