The session name suffix can be configured

Session name suffix is now configurable in services.yml.

This is especially useful for sites migrating from Drupal 7, as the old Drupal 7 session name (and its session cookie) could prevent users from logging in to the newly upgraded site (see the related issue).

Follow this procedure to add the session name_suffix:

1. Change into the sites directory (E.g., sites/default)
2. If services.yml doesn't exist already, copy default.services.yml to services.yml
3. Modify services.yml and set the name_suffix parameter to the desired value.

Also see default.services.yml for instructions:

# By default, Drupal generates a session cookie name based on the full
# domain name. Set the name_suffix to a short random string to ensure this
# session cookie name is unique on different installations on the same
# domain and path (for example, when migrating from Drupal 7).
name_suffix: ''