GitLab Migration: Improve text on PAT usage

From https://gitlab.com/gitlab-org/manage/import/discussions/-/issues/30#note_1184734852

Related to #375057 (closed)

Currently, we show the following text for creating Personal access token on the GitLab migration page:

Let's make the following improvements:

• Add the required scope (api and read_repository) to the text.
• Change the wording and advise users to consider an expiration date according to the size of the migration