ci: replace 4-core runners with ubuntu-latest (#18503)

## Summary

- Replace all `ubuntu-latest-4-cores` (paid larger runners) with
`ubuntu-latest` across CI workflows
- The free `ubuntu-latest` runner for public repos already provides **4
vCPUs + 16 GB RAM** — identical specs to the paid 4-core larger runner
- Affects 4 workflow files: `ci-server.yaml`, `ci-front.yaml`,
`ci-sdk.yaml`, `ci-zapier.yaml` (8 job definitions total, including the
10-shard integration test matrix)
- The `ubuntu-latest-8-cores` runners are intentionally **kept** for
memory-heavy jobs (frontend build, storybook build, E2E tests) where the
extra capacity (8 vCPUs, 32 GB RAM) is needed

.github/workflows/ci-front.yaml

@@ -65,7 +65,7 @@ jobs:
           key: ${{ env.STORYBOOK_BUILD_CACHE_KEY_FOR_SAVE_ACTION }}
   front-sb-test:
     timeout-minutes: 30
-    runs-on: ubuntu-latest-4-cores
+    runs-on: ubuntu-latest
     needs: front-sb-build
     strategy:
       fail-fast: false