meta: backport PR linting requires Signed-off-by

[MikeMcC399]

In #63190

https://github.com/nodejs/node/actions/runs/25552346396/job/75003001878?pr=63190 fails linting with

Error: not ok 6 signed-off-by: Commit must have a "Signed-off-by" trailer ()

According to https://github.com/nodejs/node/blob/main/doc/contributing/pull-requests.md#step-4-commit

Your commit must contain the Signed-off-by line with your name and email address as an acknowledgement that you agree to the Developer Certificate of Origin. Bot generated commits are exempt from this requirement. ... This rule does not apply to dependency updates (e.g. cherry-picks), release commits, or backport commits.

Linting is not aligned to the documentation.