Bump Visual Studio Requirement to 2017 or 2019

[ohine]

Description of bug

After merging #2774 and #649 we can no longer compile the Platform or AdminExperience in Visual Studio 2015.

The current requirements for the Platform are VS2015 and for Extensions 2013 or 2015.

https://www.dnnsoftware.com/docs/developers/requirements.html
https://dnndocs.com/content/administrators/requirements/index.html#dnn-requirements

Suggested Solution

Upgrade our required version of Visual Studio to 2017 or 2019 with the Platform v9.4 release.

[SkyeHoefling]

@ohine I agree that #2774 made the problem visible, I believe #2758 caused the problem as that enforced all platform code to compile under C# 7.0. There were many projects in the platform that were still targeting C# 6.0. #2774 used some features that are not available in C# 6.0.

I think we need to create a issue in the Admin Experience project to get everything on C# 7.0 if it isn't already there

[ohine]

Good catch @ahoefling, I totally forgot about the C# 7.0 bump.

As for AE, yes we need to get it on C#7 and .net 4.7.2. It would be nice to do the same with CDF and CK if someone has the time to submit those PRs before Friday's v9.4.0 codefreeze.

[SkyeHoefling]

PR Submitted dnnsoftware/Dnn.AdminExperience#948

[valadas]

Duplicates #2786