Allow to delete epic boards

What does this MR do?

Adds graphql mutation and destroy service for deleting epic boards.

Sample query:

mutation {
  destroyEpicBoard(input: {id: "gid://gitlab/Boards::EpicBoard/4"}) {
    epicBoard {
      id
      name
    }
    errors
  }
}

Related to #233435 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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