Update to lts 31

[tebeco]

This PR follow the logic of both :
serilog/serilog-aspnetcore#165
serilog/serilog-extensions-hosting#17

What this PR change :

• Add global.json
• Add netcoreapp3.1 as a TargetFramework (Not the best way of resolution but it's still better than doing nothing)
• Update AppVeyor image to Vs2019
• Remove linter warning in powershell script
• Use PackageIcon as PackageIconUrl is being deprecated
• Always Resolve latest dependencies at build time for Serilog

Thoughts :

As said in this PR :
serilog/serilog-extensions-hosting#17

we could also just stop support for 2.x because 3.1 is LTS <=== This would require less complexity here, and customer using runtime/Microsoft.Exntesion.* would just need to pin this dependency to the current release
Note that the fact that a customer does not want to update it own dependency is driven to "pin down" it dependencies as they need evolve.
I have no idea What is Serilog stance on that but this is what Major is for ;)

For now i've tried to do this by using cross targeting, but as said the proper solution for this lib is to drop support for any Microsoft.Extensions.xxx 2.x as 3.1 is LTS since December 2019

Side note

I have no idea who use directly this dependency, but should it be concidered to just drop net46x and net45 ?
The same way Major version and pinning works, and also since net472/net48 are supposed to be netstandard2.0 they should show be able to resolves 2.1 dependencies

[sungam3r]

This may be helpful.

And then

  public override void Load()
            {
                Load(StringToStream(_json.ToValidJson()));
            }

Still waiting for merge :(

[tebeco]

i did not saw that PR ;)
Also the double quote seems fine for all TFM, do we want to have different JSON delimiter since one works for all ?

[sungam3r]

My interest was to keep the number of changes in PR to a minimum. I achieved this with a few lines of code instead of a lot of refactoring. As a result, the tests work on both platforms. In addition, you will simply stop testing a single quote syntax. This is degradation of the test coverage.