dd: replace gcd dependency with num-integer

[wtcpython]

num-integer is already in our dependency tree; using it drops the standalone gcd crate while keeping the same behavior.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[sylvestre]

Please document the why

[oech3]

I think it is already transitive dep.

[wtcpython]

num-integer is already in our dependency tree; using it drops the standalone gcd crate while keeping the same behavior.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[cakebaker]

Thanks!