Fix problem with wrong constraint name in v2-0-stable branch (#15494)

While cherry-picking docker image changei to v2-0-test, the
value of the arg was wrongly rename (similarly to other parameters) with
`constraintis-2.0` where it should remain as `constraints`.
This is the name of constraint file to use, and it's value might
be either `constraints-no-providers`, `constraints`, or
`constraints-source-providers`.

This change restores proper default of the arg.

Fixes: #15493

Dockerfile

@@ -161,7 +161,7 @@ ENV AIRFLOW_EXTRAS=${AIRFLOW_EXTRAS}${ADDITIONAL_AIRFLOW_EXTRAS:+,}${ADDITIONAL_
 ARG CONSTRAINTS_GITHUB_REPOSITORY="apache/airflow"
 ENV CONSTRAINTS_GITHUB_REPOSITORY=${CONSTRAINTS_GITHUB_REPOSITORY}
 
-ARG AIRFLOW_CONSTRAINTS="constraints-2.0"
+ARG AIRFLOW_CONSTRAINTS="constraints"
 ENV AIRFLOW_CONSTRAINTS=${AIRFLOW_CONSTRAINTS}
 ARG AIRFLOW_CONSTRAINTS_REFERENCE=""
 ENV AIRFLOW_CONSTRAINTS_REFERENCE=${AIRFLOW_CONSTRAINTS_REFERENCE}