Target `netstandard2.0` for c#

[ShortDevelopment]

.Net-Framework v4.5 is out of suppot and the next best netframework version (v4.6.2) is already compatible with netstandard2.0.

See #1188 (comment)

Tasks

• Update internal build props
• Update samples from net45 to net46
• Update libs to netstandard2.0
• Remove conditional compilation
• Update tests
• Update docs

[chwarr]

Also, update the table in the documentation.