Add Octokit::Client#delete_organization to support org deletion

[jdennes]

This PR adds Octokit::Client#delete_organization for the recently added API endpoint to delete an org:

https://docs.github.com/rest/orgs/orgs#delete-an-organization

Resolves #1557

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Behavior

Before the change?

• The Octokit client could not delete orgs.

After the change?

• The Octokit client can delete orgs.

Other information

• Changelog item: Add Octokit::Client#delete_organization for new API endpoint to delete organizations.

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Additional info

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)
• Added the appropriate label for the given change

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes (Please add the Type: Breaking change label)
• No

If Yes, what's the impact:

• N/A

Pull request type

Please add the corresponding label for change this PR introduces:

(I can't add labels)

• Feature/model/API additions: Type: Feature

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[kfcampbell]

Thank you for contributing!