gh-gl2gh link in case of insufficient permissions needs to be changed

## Description

When using gh gl2gh (version 1.33) and having a permissions issue while migrating, the error message points to a generic docs link for GitHub > GitHub migrations. There is a separate link available for GitLab > GitHub migrations which has specific scopes for the GitHub personal access token, that deviate from the one for GitHub > GitHub. The line of code where the link is: https://github.com/github/gh-gei/blob/b9b3fa175b9bf99b429f3a628b65fd6846459aee/src/Octoshift/InsufficientPermissionsMessageGenerator.cs#L6.

What the error message looks like now:

The correct URL for GitLab is https://docs.github.com/en/enterprise-cloud@latest/migrations/using-github-enterprise-importer/migrate-from-gitlab/manage-access

## Reproduction Steps

Create a personal access token that only has "repos" scope and try to run a GitLab to GitHub migration using `gh gl2gh`.

Start in src/Octoshift/InsufficientPermissionsMessageGenerator.cs at line 6, where the insufficient-permissions link is defined. Reproduce the behavior with gh gl2gh using a personal access token with only the repos scope, then verify the error points to the GitLab-to-GitHub manage-access URL.

Written by the indexing model from the issue text.