ci: use pnpm/action-setup@v4 so CI reads the pinned pnpm version

The openapi/storybook/embeds workflows pinned pnpm 8 and cypress pinned
pnpm 6 via action-setup@v2. The repo's lockfile is lockfileVersion 9.0
(written by the pinned packageManager pnpm@10.19.0), which pnpm 8/6 cannot
read — it ignored the lockfile and --frozen-lockfile failed with
ERR_PNPM_LOCKFILE_CONFIG_MISMATCH. Drop the explicit version and let
action-setup@v4 resolve pnpm from the packageManager field, matching the
already-working publish-mcp / publish-course-app workflows.

.github/workflows/cypress.yml

@@ -18,9 +18,8 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
       - name: Set up pnpm
-        uses: pnpm/action-setup@v2.0.1
+        uses: pnpm/action-setup@v4
         with:
-          version: 6
           # Only install what we need to run the client in-action and tests against it
           # format:    `--filter <packagename>...`
           run_install: |