revert: use GitHub App token for pull-translations PR creation

[dmortal]

Summary

Reverts #2680.

• fix: use GitHub App token for pull-translations PR creation #2680 swapped pull_translations's push token from GITHUB_TOKEN to a minted GitHub App token, to work around GitHub's anti-recursion rule (pushes made with the default GITHUB_TOKEN never trigger other push-triggered workflows, so ci.yaml's required checks never ran on any lokalise-update-* branch).
• That fix worked for the push itself, but the App behind secrets.APP_ID/secrets.APP_SECRET only has contents: write (all its only other consumer, ci.yaml's derive-commit job, ever needed) — not pull_requests: write. So the very next step, actually opening the PR via the API, now fails outright with Resource not accessible by integration. This is a regression: before fix: use GitHub App token for pull-translations PR creation #2680 the PR at least opened (it just sat stuck on required checks); after fix: use GitHub App token for pull-translations PR creation #2680 it doesn't open at all.
• Fixing the App's permissions requires org/app-admin access not available here, so reverting to restore the previously-working (if imperfect) behavior rather than trade one admin-gated dependency for another.

Note: this is unrelated to check-for-manual-translations (#2671, #2674), which is fixed and unaffected by this revert.

Follow-up (manual, until a proper fix lands)

ci.yaml's required checks still won't run automatically on lokalise-update-* branches after this revert. Whoever runs "Pull Translations" next: once the PR appears, check out the branch locally, rebase/merge latest main into it, and git push with your own credentials — a normal human-authenticated push isn't subject to the anti-recursion rule and will trigger ci.yaml normally, letting the PR become mergeable.

Test plan

• Merge to main
• Close stale Lokalise PRs (chore: sync translations from Lokalise #2610) and re-run "Pull Translations" (workflow_dispatch)
• Confirm the fresh PR opens successfully (no Resource not accessible by integration)
• Confirm check-for-manual-translations still passes on it