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/upload-openapi-spec.yml

@@ -9,7 +9,6 @@ on:
 
 env:
   NODE_VERSION: '20'
-  PNPM_VERSION: '8'
 
 jobs:
   generate-and-upload-openapi:
@@ -26,9 +25,7 @@ jobs:
           node-version: ${{ env.NODE_VERSION }}
 
       - name: Setup pnpm
-        uses: pnpm/action-setup@v2
-        with:
-          version: ${{ env.PNPM_VERSION }}
+        uses: pnpm/action-setup@v4
 
       - name: Get pnpm store directory
         shell: bash