Enable interactive C# on the What's new topics

[BillWagner]

The topics under "What's new" should leverage the C# interactive window for (almost) all the samples. This issue should be split into a few PRs:

• What's new in C# 7.3
• What's new in C# 7.2
• What's new in C# 7.1
• What's new in C# 7.0
• What's new in C# 6

In addition, the What's new in C# 6 should link to the string interpolation interactive quick start.

[BillWagner]

This part of the task:

In addition, the What's new in C# 6 should link to the string interpolation interactive quick start.

Has been done in #8238