Bump Microsoft.CodeAnalysis.CSharp.Workspaces from 4.11.0 to 4.12.0 in /samples/snippets/csharp/roslyn-sdk/SyntaxQuickStart in the dotnet group

Bump Microsoft.CodeAnalysis.CSharp.Workspaces from 4.11.0 to 4.12.0 in /samples/snippets/csharp/roslyn-sdk/SyntaxQuickStart in the dotnet group #2645