fix: cherry-pick order on GitLab by reversing commmit list only once

GitLabClient already handle commit order reversing, so re-handle it in
Runner causes cherry-pick order on GitLab to be wrong.

Remove commit order handling from Runner, and instead handle difference
between GitHub and Codeberg inside GitHubClient.

Now, since the default of --bp-branch-name takes the commit list from
{GitHub,GitLab}Client directly, that means backporting branch name
on Codeberg will also be changed to have commits in the correct order
too (old to new, in line with GitHub and GitLab), which is IMO a nice
bonus.