Update the apache airflow constraints (#4001)

**Description**
Updating the apache airflow contraints to
match with the apache airflow updated version

Author: sthodimi

tox.ini

@@ -73,7 +73,7 @@ passenv =
 # Can be used to specify which tests to run, e.g.: tox -- -s
 commands =
     python -c "import os; os.system('install-custom-pkgs --install-boto-wheels')"
-    pip install 'apache-airflow==2.6.0' --constraint "https://raw.githubusercontent.com/apache/airflow/constraints-2.6.0/constraints-3.7.txt"
+    pip install 'apache-airflow==2.6.3' --constraint "https://raw.githubusercontent.com/apache/airflow/constraints-2.6.3/constraints-3.7.txt"
 
     pytest --cov=sagemaker --cov-append {posargs}
     {env:IGNORE_COVERAGE:} coverage report -i --fail-under=86