BinLogToSln hardcodes netcoreapp2.1 as TargetFramework

When synthesizing CSproj's from binlogs BinLogToSln is hardcoding `netcoreapp2.1` as the targetframework.

This will likely prevent it from using newer language features that are in the source.

We should update this targetframework - ideally to match the latest. We can't preserve the exact TargetFramework from the binlog because we are selecting the "best" which sometimes might be platform specific (eg: `net11.0-linux` for corelib) and that would break up-stack projects which are not platform specific.

We should update both the generated TargetFramework property as well as the path to the assembly copied into the output.

https://github.com/dotnet/source-indexer/blob/271fe7409b57caa69d007948f325e5e70619a610/src/SourceBrowser/src/BinLogToSln/Program.cs#L252

https://github.com/dotnet/source-indexer/blob/271fe7409b57caa69d007948f325e5e70619a610/src/SourceBrowser/src/BinLogToSln/Program.cs#L326

Ideally we could choose an option here that would get automatically updated as the project / SDK / Compiler moves forward.