fix circleci migration PAT scope docs

[XiaoPengMei]

Closes #34238

Summary

• align the CircleCI migration credential setup instructions with the environment variable reference
• clarify that the PAT requires both repo and workflow scopes

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Source	Review	Production	What Changed
actions/tutorials/migrate-to-github-actions/automated-migrations/circleci-migration.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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