Bump etcd to 3.5.x

The etcd project maintains release branches for the current version
and pevious release[1]. Because 3.6.0 was already released, 3.4.x is
no longer supported.
Bump it to the latest bug fix release of 3.5.x .

This follows the change earlier made in tempest.

[1] https://etcd.io/docs/v3.6/op-guide/versioning/

Depends-on: https://review.opendev.org/c/openstack/devstack/+/952755
Change-Id: I6bacfea93ef8bc0f007b1d9ea5f1b2898e70fb80
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

setup-etcd-env.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 set -eux
 if [ -z "$(which etcd)" ]; then
-    ETCD_VERSION=${ETCD_VERSION:-3.4.27}
+    ETCD_VERSION=${ETCD_VERSION:-3.5.21}
     case `uname -s` in
         Darwin)
             OS=darwin