Rephrase comment in setup.py (#31312)

Following up on my late comment in PR #31309, this is a try to rephrase
the comment in `setup.py` which indicates how to trigger CI build to use
"upgrade to newer dependencies".

(cherry picked from commit 492acc2)

setup.py

@@ -16,6 +16,10 @@
 # specific language governing permissions and limitations
 # under the License.
 """Setup.py for the Airflow project."""
+# To make sure the CI build is using "upgrade to newer dependencies", which is useful when you want to check
+# if the dependencies are still compatible with the latest versions as they seem to break some unrelated
+# tests in main, you can modify this file. The modification can be simply modifying this particular comment.
+# e.g. you can modify the following number "00001" to something else to trigger it.
 from __future__ import annotations
 
 import glob