[CI-476] Remove ahmadnassri/action-dependabot-auto-merge and replace with 1debit/dependabot-auto-merge-action

[TallPaulRyan]

Replaces the unsupported ahmadnassri/action-dependabot-auto-merge action and workflow with the new, Chime-internal 1debit/dependabot-auto-merge-action. We are doing this to address the deprecation of the @dependabot merge command which the current action depends on, and for which no update or support is being provided by GitHub.

The deprecation date is January 27th, 2026. If the updates provided in this pull request are not merged before this date, Dependabot PRs will no longer auto-merge, and owners will need to manually merge them.

Created by Sourcegraph batch change TallPaulRyan/SwitchDependabotAutoMerge.

[cursor]

PR Summary

Switch Dependabot auto-merge workflow to internal action

• Replace ahmadnassri/action-dependabot-auto-merge@v2 with 1debit/dependabot-auto-merge-action@v1 in .github/workflows/dependabot-auto-merge.yml
• Update inputs to use github-token, chime-dependency-bot-token, pull-request-number, and auto-merge-config-file-path
• Bump actions/checkout from v4 to v6; retain 1debit/pr-auto-merge-action@v1 gate and pull_request_target trigger

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