Bump C# language version to 11

[stephentoub]

Motivation and Context

Use the latest C# language version.

Description

Some places already had 11 set, others were still using 10. This makes it consistently 11 everywhere. C# 11 is currently the latest, with C# 12 in preview until November.

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
• [x] All unit tests pass, and I have added new tests where possible
• [x] I didn't break anyone :smile:

[MovGP0]

LGTM

[shawncal]

@stephentoub Change looks good -- some files were moved in another change, and I cannot resolve the conflicts from here. Could you please sync up?