chore: update CI dependencies to latest versions

[autopilot-project-manager]

CI Dependency Updates

This PR updates the following GitHub Actions dependencies to their latest versions:

• actions/checkout: v6 → v7
• actions/setup-python: v5 → v7
• docker/setup-buildx-action: v3 → v4
• docker/build-push-action: v6 → v7

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[github-actions]

PDF conversion benchmark

namespace=markitdown-server  memory_limit=1024Mi (1Gi)  budget_scale=1.0
(worker restarted before each book, so PEAK_MB is that book's own high-water mark)

BOOK                           SECONDS   BUDGET  PEAK_MB LIMIT_PCT     CHARS  VERDICT
-----------------------------------------------------------------------------------------
I Robot.pdf                       27.9       35      278      27%    404725  ok
Prelude to Foundation.pdf         48.5       55      296      29%    742070  ok
Foundation and Earth.pdf         149.9      180      301      29%    918690  ok
Foundation Trilogy.pdf            81.0       85      282      28%   1260682  ok

All books converted within time and memory budgets.
(per-book pod logs in /tmp/benchmark-pod-logs)

Commit 2f32344 · workflow run