It will become v4 when we changes the default.

This commit makes lockfile version 4 the default version when Cargo
tries to write to a lockfile.

The lockfile version 4 has been stabilized since 1.78.0,
and will become default in 1.83.0.
the length of transition period is pretty similar as before.

One caveat is that in other output from Cargo,
e.g., `cargo metatada`, status messages,
`SourceID` will display in the v4 URL encoded format.
This shouldn't affect the majority of Rust users,
as `SourceId` representation should be opaque to them,
unless comparing `SourceId` across different version of toolchains.