Update to RTM Serilog 3.1 dependency

Author: nblumhardt

src/Serilog.Expressions/Serilog.Expressions.csproj

@@ -18,7 +18,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Serilog" Version="3.1.0-*" />
+    <PackageReference Include="Serilog" Version="3.1.0" />
     <PackageReference Include="Nullable" Version="1.3.1" PrivateAssets="All" />
   </ItemGroup>