build: Upgrade from .NET 4.5 to 4.7.2

Support for .NET 4.7.2 was added in Mono 5.18 per
<https://www.mono-project.com/docs/about-mono/releases/5.18.0/>.

The main CVN servers in Wikimedia Cloud run on Mono 5.16
from mono-project's apt sources, but those same sources have
been updated since we provisioned the server and now provide
latest Mono 6.8 even for Debian 9 Stretch.

This change is blocked on upgrading the servers first.

Ref #13.

src/CVNBot/CVNBot.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk" DefaultTargets="restore;Clean;Build">
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net45</TargetFramework>
+    <TargetFramework>net472</TargetFramework>
     <PackageVersion>1.0.0.0</PackageVersion>
     <!-- [Restore pre-2017 behaviour]
          Output to "bin/Release", not "bin/Release/:TargetFramework". s-->