[FIX] generic_mixin: replace removed CONCURRENCY_CHECK_FIELD with '__last_update'
In Odoo 18 the BaseModel.CONCURRENCY_CHECK_FIELD constant was removed. _auto_refresh_view_on_field_changes_system still referenced it when building the set of tracked stored fields, which raised on first write to any model that inherits generic.mixin.refresh.view (including yodoo.git.source on action_sync → remove_tag → write).
Using the literal '__last_update' keeps the same intent: exclude the synthetic concurrency-check virtual field from tracked fields.