Update etcd to 3.6 (#12634)

* [etcd] Update etcd 3.5.x to 3.6.5

- Add hashes for etcd 3.6.5
- Remove etcd v2 backup task for etcd 3.6
  The etcd 3.6 removes 'etcdctl backup' command with ETCDCTL_API=2
- Downgrade etcd to 3.5 in netchecker
  The netchecker does not work with etcd 3.6 becaust it removes v2 API support (--enable-v2).
  And netchekcer does not support v3 API.

* Fix: Change etcd config to clean up v2 store before upgrading etcd to 3.6

* Bump etcd to 3.6.8

roles/etcd/tasks/install_docker.yml

@@ -23,6 +23,14 @@
     - etcd_events_cluster_setup
     - etcd_image_tag not in etcd_events_current_docker_image.stdout | default('')
 
+- name: Get currently-deployed etcd version as x.y.z format
+  set_fact:
+    etcd_current_version: "{{ (etcd_current_docker_image.stdout | regex_search('.*:v([0-9]+\\.[0-9]+\\.[0-9]+)', '\\1'))[0] | default('') }}"
+  when: etcd_cluster_setup
+
+- name: Cleanup v2 store data
+  import_tasks: clean_v2_store.yml
+
 - name: Install etcd launch script
   template:
     src: etcd.j2