[stable-only] Cap os-client-config

os-client-config became a cycle-independent project without stable and
unmaintained branches, hence it is taken from master branch always.
This broke the gate as os-client-config recently dropped support for
python version < 3.10.

This patch fixes the gate by pinning os-client-config to the previous
release that still has the support.

Change-Id: I032fe8c28a88a31039770fc031ee7d8191269561
Signed-off-by: Elod Illes <elod.illes@est.tech>
(cherry picked from commit 33cda675ac)
(cherry picked from commit 3963932059)

.zuul.yaml

@@ -11,7 +11,8 @@
     required-projects:
       - openstack/keystoneauth
       - openstack/openstacksdk
-      - openstack/os-client-config
+      - name: openstack/os-client-config
+        override-checkout: 2.1.0
 
 - project-template:
     name: openstacksdk-tox-tips
@@ -40,6 +41,7 @@
       # stable gates.
       - name: openstack/openstacksdk
       - name: openstack/os-client-config
+        override-checkout: 2.1.0
     timeout: 9000
     vars:
       devstack_localrc:
@@ -191,7 +193,8 @@
     required-projects:
       - openstack/keystoneauth
       - openstack/openstacksdk
-      - openstack/os-client-config
+      - name: openstack/os-client-config
+        override-checkout: 2.1.0
     vars:
       tox_install_siblings: true