workflows: Move npm-update to Ubuntu 20.04
workflows: Move npm-update to Ubuntu 20.04

GitHub's 18.04 additional repositories break NPM. As "ubuntu-latest" is
going to switch to 20.04 soon anyway [1], do the jump now.

[1] https://github.com/actions/virtual-environments/issues/1816

Cherry-picked from starter-kit commit 33a665925bbf

Closes #15075
garrett/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,garrett/cockpit,cockpit-project/cockpit,garrett/cockpit,martinpitt/cockpit,garrett/cockpit,cockpit-project/cockpit,garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit