Using UnmarshalWithOpts in order to ignore undefined attributes

[bendbennett]

Closes: #415

During unmarshalling of JSON, if there are fields which do not have a corresponding attribute in the schema an error is returned.

A PR on terraform-plugin-go addresses this issue.

The UnmarshalWithOpts function used in this PR will not be available to terraform-plugin-framework until the PR on terraform-plugin-go has been merged, tagged and released. Consequently, the build will fail on this PR until that time.

[bflad]

Changes here looks good -- could you add a CHANGELOG entry file? Maybe something like

```release-note:bug
internal/fwserver: Ensured `UpgradeResourceState` calls from Terraform 0.12 properly ignored attributes not defined in the schema
```

[bflad]

I think we'll also need to update the req.RawState.Unmarshal call later down in the upgrade state handlers as well (line 150 currently) and add a covering unit test for that situation.

[bflad]

Looks really good! A go mod tidy and this should be good to merge. 🚀

[bflad]

Merging so I can update and merge in some other things. 😄

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.