Require Explicit Bootstrap

Status: Completed - Backend provisioning is no longer performed automatically by default.

Terragrunt now requires explicit opt-in to bootstrap backend infrastructure. Use terragrunt backend bootstrap or pass --backend-bootstrap to a run command (e.g., terragrunt run apply --backend-bootstrap) to provision or update backend resources referenced by the remote_state block.

This strict control is no longer necessary because the default behavior already requires explicit bootstrapping.