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/templates/etcd.env.j2

@@ -25,8 +25,6 @@ ETCD_MAX_REQUEST_BYTES={{ etcd_max_request_bytes }}
 ETCD_LOG_LEVEL={{ etcd_log_level }}
 ETCD_MAX_SNAPSHOTS={{ etcd_max_snapshots }}
 ETCD_MAX_WALS={{ etcd_max_wals }}
-# Flannel need etcd v2 API
-ETCD_ENABLE_V2=true
 
 # TLS settings
 ETCD_TRUSTED_CA_FILE={{ etcd_cert_dir }}/ca.pem