Switch unified_mode to resource not provider

[EMH333]

unified_mode needs to be enabled for the resource, not provider. This correctly fixes the deprecation error in Chef 17.

Works as expected with OSL's base cookbook

Check List

• [x] All tests pass. See https://github.com/chef-cookbooks/community_cookbook_documentation/blob/master/TESTING.MD
• [ ] New functionality includes testing.
• [x] New functionality has been documented in the README if applicable
• [x] All commits have been signed for the Developer Certificate of Origin. See https://github.com/chef-cookbooks/community_cookbook_documentation/blob/master/CONTRIBUTING.MD

[lamont-granquist]

No, it isn't appropriate to do that with a true since now the behavior of the cookbook will be different on old chef-client 12.x which this cookbook still needs to support.

This needs a flag engineered specifically for this cookbook and pretty much only this cookbook which doesn't exist yet.

[lamont-granquist]

See:

https://github.com/chef/chef/pull/12502

and

https://github.com/chef/cookstyle/issues/936