Migrate from GitLink to SourceLink

[amaitland]

Currently we use GitLink, it needs upgrading to a newer version so we may as well switch to SourceLink

It supports vcxproj so we can use it for our VC++ projects

https://github.com/dotnet/sourcelink/#prerequisites-for-net-projects

[amaitland]

We can potentially look at embedding the pdb/source into the dll and not need Source link, that's another option.

dotnet/sdk#1458 (comment)

[amaitland]

For the upcoming NETCore packages the source and pdb are built into the dll as part of #3197

I upgraded to to the latest GitLink as part of #3311

Closing this now as we'll stick with GitLink for the older packages.

Will reopen if it's deemed necessary to migrate to SourceLink in the future.

[amaitland]

Now that we build with VS2019 migrating to SourceLink is desirable as GitLink doesn't support the newer PDB format.