(#2266) Update nuspec to what is being used

All Chocolatey products are now using log4net 2.0.12, however, the
nuspec file for the chocolatey.lib package still has a reference to the
older 2.0.3 version.

The decision has been taken that this doesn't warrant a new release of
this package.

nuget/chocolatey.lib/chocolatey.lib.nuspec

@@ -34,7 +34,7 @@ This is the Chocolatey Library (API / DLL) package which allows Chocolatey to be
     <releaseNotes>See all - https://docs.chocolatey.org/en-us/choco/release-notes
     </releaseNotes>
     <dependencies>
-      <dependency id="log4net" version="2.0.3" />
+      <dependency id="log4net" version="2.0.12" />
     </dependencies>
   </metadata>
 </package>