Eng work for phasing out support for .net 6 and .net 7

[claudiaregio]

• Update target framework of the design time build host from NET 6 to NET 8 (It should be aligned with minimal SDK we keep supporting.)

• Remove the logic to handle MSBuild differently for SDK older than NET 8.

• Remove MSBuild binaries shipped with CDK. Those binaries are only used with older SDKs. Removing them would reduce the download size.

[github-actions]

@claudiaregio

A fix for this issue is now available in the next pre-releases of C#DK/C#. If you'd like to try out the fix, please see these instructions to update or try pre-release versions.

[github-actions]

@claudiaregio

This issue has been fixed! Please update to the latest versions of VS Code, C# Dev Kit, and the C# extension.