**Review risk:** two-way door — changes only the pnpm version Flow
installs and runs for projects that use pnpm without pinning their own
version; reverting is a one-line change.
## Summary
Bumps the default pnpm version on `25.3` from 11.26.0 to 11.28.9, the
latest release in the pnpm 11.x line.
## What changed
- `FrontendTools.DEFAULT_PNPM_VERSION`: `11.26.0` → `11.28.9`. Projects
using pnpm without their own pinned version get 11.28.9. CI picks up the
same version, since it reads it from `FrontendTools`.
## Test summary
- Relying on CI for the pnpm install tests on this branch.
Working on behalf of Artur
Co-authored-by: totally-not-ai[bot] <290682512+totally-not-ai[bot]@users.noreply.github.com>
Co-authored-by: Artur Signell <artur@vaadin.com>