Phase out support for .NET 6 SDK & .NET 7 SDK

[claudiaregio]

End of support for .NET 6 SDK was November 12, 2024 and end of support for .NET 7 SDK was May 14, 2024. Need to phase out support for .NET 6/7 SDKs

[lifengl]

Engineering side work:

• Potentially work to notify the developer who is still using old SDK. When the NET 6/7 support ends, the code path can be updated to tell the developer why the workspace stops working.

• 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.

[claudiaregio]

Retiring-dotnet67-sdk-support.md

@lifengl File to auto-open on update above ^ , code that Brett wrote to enable this for Polyglot Notebooks located here: dotnet/interactive#2482