Update to .NET Core 3.1

Author: stulzq

CnBlogPublishTool/dotnet-cnblog.csproj

@@ -6,7 +6,7 @@
     <PackAsTool>true</PackAsTool>
     <!-- Optional performance setting - enables tiered JIT compilation-->
     <TieredCompilation>true</TieredCompilation>
-    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <ApplicationIcon>program.ico</ApplicationIcon>
     <PackageId>dotnet-cnblog</PackageId>
     <Authors>stulzq</Authors>
@@ -18,13 +18,13 @@
     <PackageLicenseUrl>https://github.com/stulzq/CnBlogPublishTool/blob/master/LICENSE</PackageLicenseUrl>
     <PackageTags>cnblogs</PackageTags>
     <PackageRequireLicenseAcceptance>true</PackageRequireLicenseAcceptance>
-    <Version>1.3.0</Version>
+    <Version>1.3.1</Version>
   </PropertyGroup>
 
   <ItemGroup>
     <PackageReference Include="MetaWeblogClient.Core" Version="1.1.0" />
-    <PackageReference Include="Newtonsoft.Json" Version="11.0.2" />
-    <PackageReference Include="Polly" Version="6.0.1" />
+    <PackageReference Include="Newtonsoft.Json" Version="12.0.3" />
+    <PackageReference Include="Polly" Version="7.2.0" />
   </ItemGroup>
 
   <ItemGroup>