Set `job_token_scope_enabled` to true by default

What does this MR do?

This MR adds a migration that sets the job_token_scope_enabled field on project_ci_cd_settings to true by default. This will mean that new projects automatically have the field enabled.

The change made by the migration will not be observable to users until the ci_scoped_job_token feature flag is turned on. It is currently off for all environments except staging.

Migration output

Migrate:

== 20210819153805 SetDefaultJobTokenScopeTrue: migrating ======================
-- change_column_default(:project_ci_cd_settings, :job_token_scope_enabled, {:from=>false, :to=>true})
   -> 0.0068s
== 20210819153805 SetDefaultJobTokenScopeTrue: migrated (0.0247s) =============

Rollback:

== 20210819153805 SetDefaultJobTokenScopeTrue: reverting ======================
-- change_column_default(:project_ci_cd_settings, :job_token_scope_enabled, {:from=>true, :to=>false})
   -> 0.0130s
== 20210819153805 SetDefaultJobTokenScopeTrue: reverted (0.0355s) =============

Issue

Feature: #328553 (closed)
Feature flag rollout: #332272 (closed)