Bump apache-airflow from 2.2.1 to 2.6.0 in /observatory-platform

Bumps [apache-airflow](https://github.com/apache/airflow) from 2.2.1 to 2.6.0.
- [Release notes](https://github.com/apache/airflow/releases)
- [Changelog](https://github.com/apache/airflow/blob/main/RELEASE_NOTES.rst)
- [Commits](apache/airflow@2.2.1...2.6.0)

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

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

observatory-platform/requirements.txt

@@ -2,7 +2,7 @@
 observatory-api
 
 # Airflow
-apache-airflow==2.2.1
+apache-airflow==2.6.0
 apache-airflow-providers-redis==2.0.1
 apache-airflow-providers-celery==2.1.0
 apache-airflow-providers-slack==4.0.0