Issue
As of November 1st 2018, any application update uploaded to the Google Play store needs to target v8.0 (API 26) or above.
解决方案
If you are seeing this warning, you should update the $(TargetFrameworkVersion) of your projects to be v8.0 or above.
如果你不是面向 Google Play 商店并希望隐藏这些警告，则可以使用 /warnasmessage:XA0114 命令行开关。 Alternatively, add
<PropertyGroup>
<MSBuildWarningsAsMessages>XA0114</MSBuildWarningsAsMessages>
</PropertyGroup>
to your .csproj file.