Fix required workflows for organization rulesets

[o-sama]

Resolves #2113

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Before the change?

• When you apply the Terraform plan, you have the following: "required_workflows": [] in state. This happens because there was no code that flattens this specific rule.

After the change?

• Added the required code to flatten this rule when flattening the ruleset's rules. When you apply the Terraform plan, you have the following in state if successful:

              "required_workflows": [
                {
                  "required_workflow": [
                    {
                      "path": "<PATH>",
                      "ref": "<REF>",
                      "repository_id": <REPO_ID>
                    }
                  ]
                }
              ]

Pull request checklist

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• [ ] Yes
• [x] No

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