[release] use `backport` NPM dist tag for backports (#79538)
### Why?
When backport releases are published, they are marked as `latest` dist
tag, which makes `next@latest` not the "latest."
x-ref: [slack
thread](https://vercel.slack.com/archives/C04LGE5SYEB/p1747984846615469)
### How?
Use `stable` dist tag for backports.
