New csproj files should use "/" as path seperator

### Description

As dotnet supports both `"/"` and `"\"`, but in linux and macos only `"/"` works dotnet should use only `"/"`.
Also a new csproj in visual Studio should use `"/"`

### Reproduction Steps

Create a new .csproj in visual studio and link to a file...

### Expected behavior

It should use `"/"`

### Actual behavior

Is uses `"\"`

### Regression?

_No response_

### Known Workarounds

_No response_

### Configuration

_No response_

### Other information

_No response_