Get rid of unsupported frameworks

[ForNeVeR]

According to the official Microsoft statements, the support for .NET Framework versions <4.5.2 has been officially ended.

Here's a Wikipedia table that summarizes the dates.

I don't think we want to support net40 that long. It's time we may migrate to 4.5.2 at least.

Note that dropping support of old frameworks is a breaking change and we should probably increment the major version if we're going to drop it.

[ForNeVeR]

Motivation: this will allow us to use IReadOnlyDictionary and some other nice things from more modern frameworks.