Update functional tests project to add target framework 3.1

[garypretty]

Spotted that this project was not targetting 3.1 when resolving an issue with the functional test build with @BruceHaley

[cleemullins]

Oddly, this is failing in the Nuget Restore, with the following error code:
##[error]No neutral-language version resource found in the file

[garypretty]

@cleemullins that is odd. I'll take a look.

[garypretty]

@cleemullins I looked into this - I believe I know what the problem is - some missing info in the project file causing an issue with NuGet restore. I'll confirm and update the PR tomorrow / Monday morning.

[garypretty]

@cleemullins I have fixed the dotnet restore issue. The test that is failing now is failing elsewhere also (https://fuselabs.visualstudio.com/SDK_v4/_build/results?buildId=104740&view=logs&j=25a9ba8f-c8bf-58db-4b6e-5f84d8ada828&t=a4096510-a441-5a7e-ff5a-a9f780242dc5&l=3119) and isn't related to any changes in this PR.

The test is related to show typing middleware and it looks like we have had issues with the timing (environment dependent) when running those tests before.

[fuselabs]

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