Bump apache-airflow-providers-slack from 9.0.0 to 9.4.0

Bumps [apache-airflow-providers-slack](https://github.com/apache/airflow) from 9.0.0 to 9.4.0.
- [Release notes](https://github.com/apache/airflow/releases)
- [Changelog](https://github.com/apache/airflow/blob/main/docker-stack-docs/changelog.rst)
- [Commits](apache/airflow@providers-slack/9.0.0...providers-slack/9.4.0)

---
updated-dependencies:
- dependency-name: apache-airflow-providers-slack
  dependency-version: 9.4.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Pipfile

@@ -8,7 +8,7 @@ aiobotocore = "==2.19.0"
 apache-airflow = "==2.10.5"
 apache-airflow-providers-amazon = "==9.2.0"
 apache-airflow-providers-sftp = "==5.0.0"
-apache-airflow-providers-slack = "==9.0.0"
+apache-airflow-providers-slack = "==9.4.0"
 apache-airflow-providers-ssh = "==4.0.0"
 boto3 = "==1.36.3"
 flask-session = "==0.5.0"