Bump apache-airflow in /tools/cloud-composer-dag-validation (#1188)

Bumps [apache-airflow](https://github.com/apache/airflow) from 2.7.3 to 2.8.0.
- [Release notes](https://github.com/apache/airflow/releases)
- [Changelog](https://github.com/apache/airflow/blob/main/RELEASE_NOTES.rst)
- [Commits](apache/airflow@2.7.3...2.8.0)

---
updated-dependencies:
- dependency-name: apache-airflow
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

tools/cloud-composer-dag-validation/requirements.txt

@@ -1,4 +1,4 @@
-apache-airflow==2.7.3
+apache-airflow==2.8.0
 apache-airflow-providers-common-sql==1.6.0
 apache-airflow-providers-ftp==3.4.2
 apache-airflow-providers-google==10.4.0