Exclude Taskflow 6.0.0 from global requirements

Taskflow 6.0.0 had a bug in the database migrations that caused
duplicate table name errors during the persistence database tables
alembic migration.
6.0.1 was released with a fix[1] for this issue. This patch excludes
6.0.0 in the global requirements to avoid the problem.

[1] https://review.opendev.org/c/openstack/taskflow/+/950967

Change-Id: Idd7c61489d4a4fd653ecd66789eeed20a998fe07

Author: johnsom

global-requirements.txt

@@ -383,7 +383,7 @@ pbr!=2.1.0  # Apache-2.0
 sherlock  # MIT
 stevedore!=3.0.0  # Apache-2.0
 tap-as-a-service  # Apache-2.0
-taskflow  # Apache-2.0
+taskflow!=6.0.0  # Apache-2.0
 tempest  # Apache-2.0
 tooz  # Apache-2.0
 tosca-parser  # Apache-2.0