Enforce lockfile schema versions

[charliermarsh]

Summary

Historically, we haven't enforced schema versions. This PR adds a versioning policy such that, if a uv version writes schema v2, then...

• It will always reject lockfiles with schema v3 or later.
• It may reject lockfiles with schema v1, but can also choose to read them, if possible.

(For example, the change we proposed to rename dev-dependencies to dependency-groups would've been backwards-compatible: newer versions of uv could still read lockfiles that used the dev-dependencies field name, but older versions should reject lockfiles that use the dependency-groups field name.)

Closes #8465.