Fix selecting scoped label in boards sidebar

What does this MR do?

When selecting a scoped label with the same scoped label key, we replace the old scoped label with the new scoped label

Follow-ups

1. I dont think there should be any logic in the mutation, this logic should move to the actions
2. Scoped labels is an ee feature and i think the actions need to be split as such when dealing with scoped labels
3. We should add a mutation that updates the label because right now were mutating too much of the object

Repro

1. Go to an issue board or epic board
2. Create 2 scoped labels with the same scoped label key
3. Apply the "other" scoped label

Resolves #334228 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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