fix(dotnet): target netcoreapp3.1 with RollForward (#3987)

Issues where encountered with SDK compatibility when the target
was set to `net6.0` even when the .NET SDK 6.0 or newer was in
use. I suspect this is because of conflicting framework dependency
requirements across various packages.

Re-target `netcoreapp3.1` while setting a roll-forward policy allowing
use of newer major-releases of the SDK when no exact match is found (this
is tested to work correctly for all supported .NET SDK releases by our
automated test suite).

packages/@jsii/dotnet-runtime-test/test/Amazon.JSII.Runtime.IntegrationTests/ComplianceTests.cs

@@ -937,7 +937,7 @@ public void OptionalAndVariadicArgumentsTest()
         [Fact(DisplayName = Prefix + nameof(JsiiAgentIsCorrect))]
         public void JsiiAgentIsCorrect()
         {
-            Assert.Equal("DotNet/" + Environment.Version + "/.NETCoreApp,Version=v6.0/1.0.0.0", JsiiAgent.Value);
+            Assert.Equal("DotNet/" + Environment.Version + "/.NETCoreApp,Version=v3.1/1.0.0.0", JsiiAgent.Value);
         }
 
         [Fact(DisplayName = Prefix + nameof(ReceiveInstanceOfPrivateClass))]

packages/@jsii/dotnet-runtime/src/Amazon.JSII.Runtime/Amazon.JSII.Runtime.csproj

@@ -2,7 +2,8 @@
   <Import Project="../NuGet.Metadata.props" />
 
   <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <RollForward>Major</RollForward>
     <PackageId>Amazon.JSII.Runtime</PackageId>
     <Title>.NET Runtime for JSII</Title>
     <PackageIcon>icon.png</PackageIcon>

packages/@jsii/dotnet-runtime/src/Amazon.JSII.Runtime/Services/TypeCache.cs

@@ -202,7 +202,7 @@ void CacheTypes(Assembly assembly)
                     }
                 }
 
-                return e.Types;
+                return e.Types ?? Array.Empty<Type?>();
             }
         }
     }

packages/jsii-pacmak/lib/targets/dotnet.ts

@@ -18,7 +18,7 @@ import { toReleaseVersion } from './version-utils';
 
 import { TargetName } from '.';
 
-export const TARGET_FRAMEWORK = 'net6.0';
+export const TARGET_FRAMEWORK = 'netcoreapp3.1';
 
 /**
  * Build .NET packages all together, by generating an aggregate solution file

packages/jsii-pacmak/lib/targets/dotnet/filegenerator.ts

@@ -112,6 +112,8 @@ export class FileGenerator {
     propertyGroup.ele('Nullable', 'enable');
     propertyGroup.ele('SymbolPackageFormat', 'snupkg');
     propertyGroup.ele('TargetFramework', TARGET_FRAMEWORK);
+    // Transparently rolll forward across major SDK releases if needed
+    propertyGroup.ele('RollForward', 'Major');
 
     const itemGroup1 = rootNode.ele('ItemGroup');
     const embeddedResource = itemGroup1.ele('EmbeddedResource');