[release/3.1] Update dependencies from dotnet/corefx (#26508)

[release/3.1] Update dependencies from dotnet/corefx (#26508)

* Update dependencies from https://github.com/dotnet/corefx build 20190903.6

- Microsoft.NETCore.Platforms - 3.0.0-rc1.19453.6
- Microsoft.Private.CoreFx.NETCoreApp - 4.7.0-rc1.19453.6

* Update dependencies from https://github.com/dotnet/corefx build 20190904.8

- Microsoft.NETCore.Platforms - 3.0.0-rc1.19454.8
- Microsoft.Private.CoreFx.NETCoreApp - 4.7.0-rc1.19454.8

* Update dependencies from https://github.com/dotnet/corefx build 20190906.2

- Microsoft.NETCore.Platforms - 3.1.0-preview1.19456.2
- Microsoft.Private.CoreFx.NETCoreApp - 4.7.0-preview1.19456.2

* changing the target framework for tests to 3.1

* Update dependencies from https://github.com/dotnet/corefx build 20190906.20

- Microsoft.NETCore.Platforms - 3.1.0-preview1.19456.20
- Microsoft.Private.CoreFx.NETCoreApp - 4.7.0-preview1.19456.20

* Update dependencies from https://github.com/dotnet/corefx build 20190907.5

- Microsoft.NETCore.Platforms - 3.1.0-preview1.19457.5
- Microsoft.Private.CoreFx.NETCoreApp - 4.7.0-preview1.19457.5

* Update dependencies from https://github.com/dotnet/corefx build 20190909.13

- Microsoft.NETCore.Platforms - 3.1.0-preview1.19459.13
- Microsoft.Private.CoreFx.NETCoreApp - 4.7.0-preview1.19459.13

* 3.0 -> 3.1