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chore: update GitHub Actions to latest versions

- Bump actions/checkout v4→v7, actions/setup-go v5→v6,
  actions/cache v4→v6, golangci/golangci-lint-action v6→v9
- Migrate .golangci.yaml to v2 config format (golangci-lint v2);
  gofumpt is now a formatter, and the ST10xx stylecheck rules that
  v2 folds into staticcheck are disabled to keep error strings and
  identifier names aligned with upstream Playwright
- Fix the issues the v2 linters surface: unchecked errors, a
  tagged-switch and De Morgan simplification, ReplaceAll, and two
  ineffective break statements in a switch
- Add concurrency groups to all workflows (cancel-in-progress for
  CI, disabled for the docs deploy)