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/handlers/backup.yml

@@ -34,6 +34,7 @@
   when:
     - etcd_data_dir_member.stat.exists
     - etcd_cluster_is_healthy.rc == 0
+    - etcd_version is version('3.6.0', '<')
   command: >-
     {{ bin_dir }}/etcdctl backup
       --data-dir {{ etcd_data_dir }}