fix(dotnet): target netcoreapp3.1 with RollForward

[RomainMuller]

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).

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[RomainMuller]

The integration test failure on this run is spurious but cannot be avoided (this is due to our rolling back the TargetFramework, while we have existing artifacts published that only support net6.0. A release will fix that, but we need to skip over the integration tests this time around.

[mergify]

Thank you for contributing! ❤️ I will now look into making sure the PR is up-to-date, then proceed to try and merge it!

[mergify]

Merging (with squash)...

[cplee]

This change is causing issues with aws/aws-cdk#23921

I'm unable to package a CDK construct anymore: https://github.com/cdklabs/cdk-ecs-codedeploy/actions/runs/4347270960/jobs/7594403891

[biffgaut]

Our packaging process has broken as well -

Build Logs

[cplee]

I was able to work through it with latest version of CDK released yesterday (2.68.0) and latest JSII (1.77.0)

[biffgaut]

Upgrading to 2.68.0 fixed it for us as well.