pin Slack SDK to 3.18.1 to avoid failing issue
# What does this PR do?

Currently CI Slack reports failed to be sent due to an error
```bash
The server responded with: {'ok': False, 'error': 'invalid_blocks_format'}
```
It happens since `slack-sdk-3.18.2`. This PR pin `slack-sdk-3.18.1` so we can receive the reports.
_The documentation is not available anymore as the PR was closed or merged._