[Issue #6757] Migrate terraform from DynamoDB locks to S3 native locks

[sean-navapbc]

Summary

• Migrate all Terraform backend configurations from DynamoDB state locking to S3 native locking
• DynamoDB locking is deprecated as of Terraform 1.7+: https://developer.hashicorp.com/terraform/language/backend/s3#enabling-dynamodb-state-locking-deprecated
• Updated all 43 .tfbackend files to use use_lockfile = true instead of dynamodb_table
• Made DynamoDB table creation optional in terraform-backend-s3 module

Changes

1. Backend configuration files: Replaced dynamodb_table = "..." with use_lockfile = true in all .tfbackend files
2. terraform-backend-s3 module:
   • Added enable_dynamodb_lock_table variable (default: false)
   • Made DynamoDB table and KMS key creation conditional
   • Updated README with S3 native locking documentation
3. accounts/main.tf: Set enable_dynamodb_lock_table = true to continue managing the existing DynamoDB table

Migration Notes

• The existing DynamoDB table is preserved but no longer used for locking
• S3 native locking uses conditional writes to the state file instead of DynamoDB
• The DynamoDB table can be manually deleted in a future cleanup PR once this migration is verified

Test plan

• Verify terraform init works with new backend configuration
• Verify terraform plan/apply works with S3 native locking
• Verify CI/CD terraform apply is working without DynamoDB locking

Closes #6757