fix: cap npm at 11.x so the publish job works on node 20

[limitofzero]

Summary

The Publish NPM Package job has been failing since August, so @cowprotocol/cms is stuck at
0.13.0 on npm while this repo is at 0.15.0.

Two constraints collided:

• NODE_VERSION is pinned to lts/iron (Node 20) because Strapi 4.26.1 doesn't support Node 22.
• npm@latest is now npm 12, which declares engines.node: "^22.22.2 || ^24.15.0 || >=26.0.0".

So the npm install -g npm@latest step aborts with EBADENGINE before the job ever reaches
yarn install or publish-lib.sh:

npm error notsup Not compatible with your version of node/npm: npm@12.0.2
npm error notsup Required: {"node":"^22.22.2 || ^24.15.0 || >=26.0.0"}
npm error notsup Actual:   {"npm":"10.8.2","node":"v20.20.2"}

Capping the range at ^11 resolves to 11.19.1, whose engines.node is
"^20.17.0 || >=22.9.0" — satisfied by the runner's v20.20.2 — while staying above the >= 11.5.1
that trusted publishing needs. The step keeps doing its job; it just stops following npm across a
major that Node 20 can't run.

Unblocks cowprotocol/cowswap#7846, which needs the CMS collection types
from 0.15.0.