Update to Validation 2.4.18

Fixes NuGet package restore warnings

Author: AArnott

src/NerdBank.GitVersioning/NerdBank.GitVersioning.csproj

@@ -11,7 +11,7 @@
     <PackageReference Include="LibGit2Sharp" Version="0.25.2" PrivateAssets="none" />
     <PackageReference Include="Newtonsoft.Json" Version="11.0.2" />
     <PackageReference Include="System.Diagnostics.Tools" Version="4.3.0" Condition=" '$(TargetFramework)' == 'netcoreapp2.0' " />
-    <PackageReference Include="Validation" Version="2.3.7" />
+    <PackageReference Include="Validation" Version="2.4.18" />
     <PackageReference Include="Nerdbank.GitVersioning.LKG" Version="1.6.20-beta-gfea83a8c9e" />
   </ItemGroup>
   <ItemGroup>