Change default of `restrict_user_defined_variables` to `true` behind FF

Related to: #502382

Today restrict_user_defined_variables is set to false by default, meaning that all developers are allowed to pass pipeline variables when creating pipelines. This default setting violates the least privilege principle.

What does this MR do and why?

Set restrict_user_defined_variables to true by default.

Use pipeline_variables_minimum_override_role from the root namespace settings (pipeline_variables_default_role).

Default pipeline_variables_default_role to developer in namespace settings.

Behind an FF change_namespace_default_role_for_pipeline_variables disabled by default:

1. disabled - pipeline_variables_default_role = developer
2. enabled - pipeline_variables_default_role = no_one_allowed

The idea is to:

1. Begin the rollout with the default developer role, enabling both restrict_user_defined_variables and pipeline_variables_minimum_override_role.
2. In later milestones, use namespace migrations to enforce stricter limits, first changing the role to maintainer and eventually to no_one_allowed.

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