Remove stale Aspire 9.x references from getting started docs

[davidfowl]

Problem

Several get-started pages still refer to Aspire 9.x directly. These references feel stale in getting-started content and should be removed or rewritten with current, version-neutral wording unless the version callout is still required.

Current references

• src/frontend/src/content/docs/get-started/github-codespaces.mdx
  • Intro says "With Aspire 9.1"
  • Follow-up says "Before Aspire 9.1"
  • Comparison section says Aspire 9.1 enhanced Codespaces/Dev Containers support
• src/frontend/src/content/docs/get-started/dev-containers.mdx
  • Intro says "With Aspire 9.1"
  • Follow-up says "Before Aspire 9.1"
  • Comparison section says "With the release of Aspire 9.1"
• src/frontend/src/content/docs/get-started/csharp-service-defaults.mdx
  • Note says "Starting with Aspire 9.2"
• src/frontend/src/content/docs/get-started/add-aspire-existing-app.mdx
  • Note says "Unlike the deprecated AddNpmApp API in Aspire 9"

Expected outcome

• Remove or rewrite stale Aspire 9.x references in get-started docs.
• Prefer current/product-centric wording over historical release framing in getting-started content.
• Keep a version-specific note only when it is still necessary to explain current behavior.