tutorial: remove warn parameter from 'shell' ansible module

This parameter is removed in ansible-core 2.14 [1][2] and an error is raised
during tutorial run.

FAILED! => {"changed": false, "msg": "Unsupported parameters for
(ansible.legacy.command) module: warn.

[1] https://github.com/ansible/ansible/issues/77394
[2] https://github.com/ansible/ansible/pull/77411

Change-Id: I7ee86f019eeac14ddb22abc7924d0a10b051750e

playbooks/tutorial/quick-start.yaml

@@ -50,7 +50,6 @@
   shell:
     chdir: "{{ workspace }}/zuul-config/"
     executable: /bin/bash
-    warn: false
     cmd: |
       {{ ssh_agent.stdout }}
       git add playbooks zuul.d/jobs.yaml

playbooks/tutorial/roles/setup-tutorial/tasks/main.yaml

@@ -126,7 +126,6 @@
   shell:
     chdir: "{{ workspace }}/zuul-config/"
     executable: /bin/bash
-    warn: false
     cmd: |
       {{ ssh_agent.stdout }}
       git add zuul.d