Fix required workflows for organization rulesets

[o-sama]

Resolves #2113

---

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
• No

---