Remove the pipeline: variables section from azure.yaml

As of azd version 1.17.0, the `pipeline:` `variables:` section in `azure.yaml` is no longer needed.
The Azure Developer CLI will auto-detect the needed variables from `main.parameters.json`.

Please update this repository's `azure.yaml` by:

1. Removing the entire `pipeline:` block (including nested `variables:` entries).
2. Adding (or updating) a `requiredVersions` section to enforce a modern azd version:

```yaml
requiredVersions:
azd: ">= 1.17.0"
```
3. If the file uses a `metadata.template` version, bump that version to indicate the change.
4. Re-run `azd up` or your standard CI workflow to ensure everything still provisions correctly.

Rationale:
- Simplifies configuration (variables now inferred automatically)
- Reduces duplication and risk of drift
- Aligns with current azd best practices

After making the change, commit and open a PR referencing this issue.

## Details

**Pattern:** `pipeline:`

**File:** `azure.yaml`

### Found matches:

**Line 39:**
```
pipeline:
```

**Repository:** https://github.com/Azure-Samples/rag-postgres-openai-python

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*This issue was automatically created by the GitHub Repo Maintainer Agent.*