Remove `Cargo.lock` file exception from `.gitignore`

[alejandro-vaz]

As recommended by rust-lang/cargo#12382.

Lockfiles should be committed even in libraries.

Affects v1 and v2 branch.

[alejandro-vaz]

v2 in #421

[alejandro-vaz]

v1 won't implement this due to Cargo.lock version incompatibility

v2 is safe, lockfile v4 was made the default in 1.83

can't be implemented into v1 because it breaks the CI workflow

[alejandro-vaz]

merged