Reduce dependency on MEL -> MELA

Ran across this while working on open-feature/dotnet-sdk-contrib#127,
but tldr; libraries don't need the implementation package, so reduce
to the abstractions package to remove transitive dependencies on:

- Microsoft.Extensions.DependencyInjection
- Microsoft.Extensions.Options
- (and a few others depending on the TFM)

Signed-off-by: Austin Drenski <austin@austindrenski.io>

src/OpenFeature/OpenFeature.csproj

@@ -7,7 +7,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.Extensions.Logging" Version="$(MicrosoftExtensionsLoggerVer)" />
+    <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" Version="$(MicrosoftExtensionsLoggerVer)" />
   </ItemGroup>
 
   <ItemGroup>
@@ -23,4 +23,4 @@
     <None Include="../../README.md" Pack="true" PackagePath="/" />
   </ItemGroup>
 
-</Project>
+</Project>