Migrate restrict_user_defined_variables to true

What does this MR do and why?

This adds a Batched Background Migration that migrates the CI/CD setting restrict_user_defined_variables: false to true while also keeping the behavior backwards compatible by also updating pipeline_variables_minimum_override_role: :developer.

This migrations is necessary in order to deprecate restrict_user_defined_variables in %18.0. In order to do that we need to have restrict_user_defined_variables: true everywhere and solely rely on pipeline_variables_minimum_override_role.

DB migrations

➜ rails db:migrate                             
main: == [advisory_lock_connection] object_id: 134040, pg_backend_pid: 28034
main: == 20250220113009 QueueEnableRestrictUserDefinedVariables: migrating ==========
main: == 20250220113009 QueueEnableRestrictUserDefinedVariables: migrated (0.0475s) =

main: == [advisory_lock_connection] object_id: 134040, pg_backend_pid: 28034

➜ rails db:rollback:main VERSION=20250220113009
main: == [advisory_lock_connection] object_id: 133620, pg_backend_pid: 30009
main: == 20250220113009 QueueEnableRestrictUserDefinedVariables: reverting ==========
main: == 20250220113009 QueueEnableRestrictUserDefinedVariables: reverted (0.0938s) =

main: == [advisory_lock_connection] object_id: 133620, pg_backend_pid: 30009

References

Related to Migrate projects using pipeline variables to fo... (#514243 - closed)

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