build(deps): bump log4net from 2.0.14 to 2.0.15 in /src

Bumps [log4net](https://github.com/apache/logging-log4net) from 2.0.14 to 2.0.15.
- [Release notes](https://github.com/apache/logging-log4net/releases)
- [Changelog](https://github.com/apache/logging-log4net/blob/master/doc/ReleaseInstructions-legacy.txt)
- [Commits](apache/logging-log4net@rel/2.0.14...rel/2.0.15)

---
updated-dependencies:
- dependency-name: log4net
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

src/Splat.Log4Net/Splat.Log4Net.csproj

@@ -12,7 +12,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="log4net" Version="2.0.14" />
+    <PackageReference Include="log4net" Version="2.0.15" />
 
     <!-- Override old versions provided in the .NET Standard 1.3 package -->
     <PackageReference Include="System.AppContext" Version="4.3.0" />