Remove default of `1` for `cell.id`

Motivation

Why do we want to remove the default ? @OmarQunsulGitlab

If the GitLab Rails application has the setting of cell.id set, then it means that the application is part of a Cells cluster. We want to revert non-cell instances to the prior-cells configuration, there nothing is set. That means self-managed instances will never have cell.id set.

It's safe to change this back to nil, because it won't affect any existing functionality.

Context

• Default cell.id is set to 1 now. https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/gitlab.yml.example#L1289-1292. https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/initializers/1_settings.rb#L1046-1047. We need to set the default value to nil instead.

Action items

• Resolve impact on app/models/concerns/token_authenticatable_strategies/routable_token_generator.rb. Consider the production rollout for the legacy cell.
• Remove default of 1 for cell.id
• Do we need validation for the value of cell.id to make sure it does not clash with other cells ? How ? > #510677 (moved)