Remove underscore from _UseManagedNtlm

[am11]

Now that it's documented https://learn.microsoft.com/dotnet/api/system.net.networkcredential#remarks and no longer an internal property, lets make it consistent with rest of the config options (without the leading underscore).

[am11]

cc @filipnavara, @wfurt, @rzikm

[filipnavara]

I'm torn about this. Technically you are right. It should have never made the documentation with the current name and the ship has sailed on that. Changing the name now will make the documentation invalid and also any usage of the property in the wild. I still consider the option to be an unsupported compatibility hack which happens to be useful for testing or workarounds. I'll defer the decision to others, but thanks for tagging me and keeping me in the loop!

[filipnavara]

I did a quick GitHub search and didn't find any projects using the _UseManagedNtlm property aside from dotnet/sdk, dotnet/runtime and their forks.

There's a usage in dotnet/runtime that would have to be updated if this goes through:
https://github.com/dotnet/runtime/blob/f5c73447ca9dcb3407d0143829bbf708c04170c1/src/tools/illink/src/ILLink.Tasks/build/Microsoft.NET.ILLink.targets#L53-L55

[wfurt]

I'm OK either way. We can always document it as breaking change to make it more visible. I would think the need for this is small now....

[rzikm]

I am on the same page as wfurt.