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GH-41282: [Dev] Always prompt next major version on merge script if it exists (#41305)

### Rationale for this change

When we created the `16.1.0` milestone the merge script started prompting it instead of `17.0.0` we want to default to the next major release.

### What changes are included in this PR?

Update archery logic to default to major versions online.

### Are these changes tested?
I've tested locally and now it defaults to `17.0.0`:

```
Enter fix version [17.0.0]: 
```

### Are there any user-facing changes?
No
* GitHub Issue: #41282

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>