## 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 https://github.com/astral-sh/uv/issues/8465.
