Resolution comment length bug when Secret Scanning alerts

[theztefan]

Description

With the PR #1337 (that addresses #1334) we introduced a small bug when the new comment is longer then the allowed limit of the resolution comment of a secret scanning alert.

Reproduction Steps

Run gh gei migrate-secret-alerts with a repository that has a resolved comment with a length >270.

Proposed improvement

With new comment = concat original author + original comment, if the length of new comment > 270 we basically have two options:

1. Trim the end of new comment and then migrate
2. Migrate original comment (excluding the original author information)