Remove deprecated TextCompletionBackend call from Getting Started workbook

[jdunnaway]

Motivation and Context

Description

Pinned the version of the workbook, included the kernel configuration namespace, updated to the appropriate method calls.

Contribution Checklist

• [ x] The code builds clean without any errors or warnings
• [ x] The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• [x ] The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• [ n/a ] All unit tests pass, and I have added new tests where possible
• [ x] I didn't break anyone 😄

[alexchaomander]

Hi @jdunnaway! Thanks for creating this! We already have a PR for this at #221. Can we consolidate this effort over there?

[jdunnaway]

Ah! Not sure how I missed that. Thanks, @alexchaomander , I'll close this PR as the same edits are being tracked there.