three errors at the first time I open the project

#### Issue details
Hi, bro. I'm a green hand so I have some questions to solve. I had download the lastest project with git fully, and I had also installed VS2017 and selected those necessary. But the IDE found that there are still 3 errors :

[1] ERROR NETSDK1045 Your Net SDK does not support setting**. Net core 3.1** as a target. Please target. Net core 2.2 or earlier, or use a. Net SDK version that supports**. Net core 3.1.** px4uploader C:\Program Files (x86)\dotnet\sdk\2.2.110\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets 137
[2] ERROR NETSDK1045 Your Net SDK does not support setting**. Net core 3.1** as a target. Please target. Net core 2.2 or earlier, or use a. Net SDK version that supports**. Net core 3.1.** Ntrip C:\Program Files (x86)\dotnet\sdk\2.2.110\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets 137
[3] ERROR NETSDK1045 Your Net SDK does not support setting**. Net standard 2.1** as the target. Please set. Net standard 2.0 or lower as the target, or use a. Net SDK version that supports**. Net standard 2.1.** wasm C:\Program Files (x86)\dotnet\sdk\2.2.110\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.TargetFrameworkInference.targets 150

I know from the Internet that **.net core 3.1** doesnot support VS2017. I donot know what should I do to make "0 error". If you know it, please tell me. Thanks very much!
Know little about Markdown, sorry for the format.

#### Version
Mission Planner Ground Control Station (c# .net) master

#### Platform
[ 1 ] VS2017 Community

Start by checking the project target-framework settings and the installed .NET SDK reported by the VS2017 environment. Compare the requirements for the px4uploader, Ntrip, and wasm projects with the reported SDK 2.2.110. Done means the supported setup or required project settings are documented and the solution builds without these three errors.

Written by the indexing model from the issue text.