Adjust TFLint blocking policy and fix deprecated interpolation warnings

[lweberru]

Background

In the current CI linting setup, some TFLint warnings should not block the pipeline while others should still fail the job.

Requirements

• terraform_unused_declarations should be reported but must not fail the job
• terraform_required_providers should also be report-only for now, because we plan to migrate fully to STACKIT and away from HashiCorp Vault
• all other warnings should remain blocking
• fix existing terraform_deprecated_interpolation warnings in the bucket-related files

Acceptance Criteria

• Workflow still shows unused_declarations and required_providers, but does not block on them
• other TFLint warnings still block the workflow
• interpolation-only expressions in the affected files are removed