Clarify if version labels for subgraph deployments have to be unique

[PaulRBerg]

Here: https://thegraph.com/docs/en/subgraphs/developing/deploying/using-subgraph-studio/#deploying-a-subgraph

It's not mentioned if the version labels have to be unique. There are two possible behaviors:

1. Deployments for particular version labels are immutable once made
2. Deployments for particular version labels are NOT immutable

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Edit: it looks like the correct answer is 1, version labels are immutable:

✖ Failed to deploy to Graph node https://api.studio.thegraph.com/deploy/: Version label already exists.