[breaking] remove `cmd.run` string signature

[bearomorphism]

I'm thinking whether we should just remove it and enforce the user to pass a list instead. probably not the best UX 🤔

Originally posted by @Lee-W in #1941 (comment)

[bearomorphism]

Removing the cmd.run(cmd: str | Sequence[str]) string overload is a breaking change — any external caller using the string form (including commitizen-action and several third-party plugins) would break on upgrade. Master still raises a DeprecationWarning on the string form (commitizen/cmd.py:75-92), so the deprecation is in motion but the actual removal needs to land on a major boundary.

Suggest scheduling this for v5 (#1481) and closing this issue in favour of a v5 checklist item. Surfaced via the round-2 triage in #1965.