Move duplicated ISO timestamp parser to shared datetime_utils

[statxc]

Description

• Extract _parse_iso() from validator/issue_discovery/repo_scan.py into validator/utils/datetime_utils.py as parse_iso()
• Import the shared function in repo_scan.py

[anderdc]

Closing this one out:

• _parse_iso isn't duplicated anywhere, it only exists in repo_scan.py
• parse_github_timestamp_to_cst in datetime_utils.py is a different function (converts to CST, not just parsing)
• Moving a single-use private helper to a shared module is premature, adds an import hop for no benefit
• If we need parse_iso elsewhere later, that's the time to extract it