Mark Golang hosting package as obsolete

[DavidGarton8]

Closes #1315

Marks the CommunityToolkit.Aspire.Hosting.Golang package as obsolete in favor of Aspire.Hosting.Go.

This change:

• Adds Obsolete attributes to the legacy extension type, public extension methods, and legacy resource types
• Updates obsolete messages to direct users to Aspire.Hosting.Go
• Updates the package README to state that this package is obsolete and that new applications should use Aspire.Hosting.Go
• Suppresses CS0618 warnings in test and example projects that intentionally continue using the legacy APIs

No functional behavior was changed.

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• New integration
  • Docs are written
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

This PR intentionally does not introduce a replacement implementation in this package. It only marks the existing package surface as obsolete and updates documentation to direct users to Aspire.Hosting.Go.

This PR should be merged when 13.4.0 is released, so the obsoletion guidance can direct users to Aspire.Hosting.Go once that package is publicly available.

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/CommunityToolkit/Aspire/main/eng/scripts/dogfood-pr.sh | bash -s -- 1323

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/CommunityToolkit/Aspire/main/eng/scripts/dogfood-pr.ps1) } 1323"