Add more platforms to supported .NET Standard platforms

[jzeferino]

Add more platforms to supported .NET Standard platforms

I wan't to add this platforms to the .NET Standard supported platforms here:

• Xamarin.iOS 10.14
• Xamarin.Android 7.5
• Xamarin.Mac 3.8
• Mono 5.4

Related to #3813

Suggest reviewers

@rpetrusha, @mairaw

Can I PR it?

[mairaw]

@terrajobst any thoughts on this?

I'd probably future-proof the list as well. So we could list:
Xamarin.iOS 10.0 and later versions
Xamarin.Android 7.0 and later versions
Xamarin.Mac 3.0 and later versions
Mono 4.6 and later versions

Note that I changed the versions to be ones supported since .NET Standard 1.0 and not the 2.0 only.

[jzeferino]

@mairaw do you want me to change my PR?

[mairaw]

I don't see any open PRs @jzeferino. An I missing something?

[jzeferino]

Sorry @mairaw I was wrong, I din't created the PR yet. I will create it.

[philippgille]

@jzeferino: I followed your advice and added more platforms to my PR. I added a "Close #3814" in the commit message, because the PR solves this issue.