Use newer token session route

[hanoii]

rest.routing.yml:

# @deprecated This route is deprecated, use the system.csrftoken route from the
# system module instead.
# @todo Remove this route in Drupal 9.0.0.
rest.csrftoken:
  path: '/rest/session/token'

system.routing.yml

system.csrftoken:
  path: '/session/token'

Not urgent, but worth doing.

[signalpoint]

3+ years later, thank you @hanoii