Fix: Add missing 3.1 5.0x targets in build.

Author: jafin

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        dotnet: ['6.0.x','7.0.x']
+        dotnet: ['3.1.x','5.0.x','6.0.x','7.0.x']
     steps:
       - uses: actions/checkout@v3
       - name: Setup dotnet

PdfSharpCore.Test/PdfSharpCore.Test.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net5.0;net6.0</TargetFrameworks>
+    <TargetFrameworks>netcoreapp3.1;net5.0;net6.0</TargetFrameworks>
     <CheckEolTargetFramework>false</CheckEolTargetFramework>
     <IsPackable>false</IsPackable>
   </PropertyGroup>