Use `upload-time` rather than `upload_time` in `uv.lock` (#13176)
## Summary
In https://github.com/astral-sh/uv/pull/12968, we added support for
upload time to `uv.lock`, but stylized as `upload_time`. The other keys
in `uv.lock` use kebab casing, as in common in Python formats, so this
really should've been `upload-time`. I want to change it ASAP to
minimize churn for users. Any users that already upgraded will of course
experience churn in their files a second time. But if we don't change it
now, we'll only increase the surface area of affected users.
So, this PR uses `upload-time` instead, but continues reading
`upload_time` to make it non-breaking.
