[stable-only] Pin tox<4 in run-both.yaml playbook also

We pinned tox<4 in ensure-tox pre.yaml playbook
- https://review.opendev.org/q/I9a138af94dedc0d8ce5a0d519d75779415d3c30b

but did not realize that it is used in run-both.yaml playbook
also where we should pin too.

Change-Id: I4407f7036e3fd51ac79f68791151c3f9cd03bd01
(cherry picked from commit 689b399e3a)
(cherry picked from commit ff4cd115cc)
(cherry picked from commit f895418f86)
(cherry picked from commit 7fce80880f)

playbooks/tox/run-both.yaml

@@ -6,6 +6,12 @@
       bindep_profile: test
       bindep_dir: "{{ zuul_work_dir }}"
     - test-setup
-    - ensure-tox
+    # NOTE(gmann): Pinning tox<4.0.0 for stable/zed and lower. Tox 4.0.0
+    # released after zed was released and has some incompatible changes
+    # and it is ok not to fix the issues caused by tox 4.0.0 in stable
+    # beanches jobs. We can continue testing the stable/zed and lower
+    # branches with tox<4.0.0
+    - role: ensure-tox
+      ensure_tox_version: "<4"
     - get-devstack-os-environment
     - tox