Fix CI: drop pnpm/action-setup `version: 11` (conflicts with packageManager field)

[yomybaby]

Background

Since the pnpm v11 migration (FR-2840 / PR #7307), the root package.json declares "packageManager": "pnpm@11.0.8". Several GitHub Actions workflows additionally pass version: 11 to pnpm/action-setup@v5, and that action errors out when both are present:

Error: Multiple versions of pnpm specified:
  - version 11 in the GitHub Action config with the key "version"
  - version pnpm@11.0.8 in the package.json with the key "packageManager"
Remove one of these versions to avoid version mismatch errors like ERR_PNPM_BAD_PM_VERSION

As a result, every PR's react-vitest (and other workflow jobs) fail before vitest ever runs. Example: https://github.com/lablup/backend.ai-webui/actions/runs/25669252851/job/75349895261

Fix

Drop the version: 11 (or version: '11') input from pnpm/action-setup@v5 calls in all workflows and rely on packageManager as the single source of truth.

Affected workflows:

• .github/workflows/vitest.yml (3 occurrences — the active CI blocker)
• .github/workflows/publish-backend.ai-ui.yml
• .github/workflows/publish-backend.ai-docs-toolkit.yml
• .github/workflows/package.yml
• .github/workflows/weekly-merge-branch-lockfiles.yml
• .github/workflows/e2e-healer.lock.yml
• .github/workflows/e2e-watchdog.lock.yml

Acceptance Criteria

• No workflow file passes version: 11 to pnpm/action-setup@v5.
• react-vitest job on a fresh PR succeeds (or fails for a real test reason, not the action-setup error).

JIRA Issue: FR-2861