Clarify GitHub App auth guidance in the ARC quickstart

[Syqens]

Summary

• add a short note near the PAT-based ARC quickstart example
• recommend GitHub App authentication for repository- and organization-level runners
• link readers to the existing ARC authentication guidance
• clarify that enterprise-level runners still require PAT classic authentication

Why

Issue #37288 points out that the quickstart currently highlights a PAT-based setup even though the docs already support GitHub App authentication. This keeps the quickstart small while guiding readers to the recommended authentication path where supported.

Testing

• content-only change
• verified the wording against the existing ARC authentication docs

[github-actions]

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Table of review links

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The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
actions/how-tos/manage-runners/use-actions-runner-controller/deploy-runner-scale-sets.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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