Because we might use dependencies soon that require this framework.
And anyway these days the .NET world is mostly a hybrid between
.NETStandard/.NETCore libs and .NET4.6 ones, not older than that.
People are reporting that under some circumstances, an infinite
loop could happen when TLSharp tries to handle a reconnection to
a different DC, if the DC instructed to be used is the same as
the one that was used in the last connection.
Not sure how could this happen (although the analysis present in
this github issue [1] might help understand it), but this commit
helps to make TLSharp fail fast (with an exception) instead of an
infinite loop from now on, which will help avoiding people file
issues such as [2] and [3] and instead maybe file a proper bug
report easier to understand, to try to fix the underlying root
cause.
[1] https://github.com/sochix/TLSharp/issues/719
[2] https://github.com/sochix/TLSharp/issues/803
[3] https://github.com/sochix/TLSharp/issues/839
This change is good because:
a) It's better to target a lower target framework version if
the project doesn't necessarily depend on the new features
of the newer versions (so, bigger target audience).
b) It lets compile the project with implementations of the
.NET Framework that are not compatible with 4.5.2. For example,
after this change I can successfully build TLSharp with the
Mono v4.2.1 that comes in my Ubuntu Linux 16.04.1.
