etcd-website: Update migration process guidance with etcdctl v3.4

[wendy-ha18]

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This PR uses to update the document for migrate etcd v2 to etcd v3 at this issue: https://github.com/etcd-io/website/issues/798

• [x] Add prerequisites.
• [x] Define all variables and flags used in the examples in a bulleted list.
• [x] Describe process of migration.
• [x] Add note to guide user that etcdctl migrate command was removed in etcd v3.5.0 (pull/12971). They must use etcdctl v3.4 or earlier to perform the migration (CHANGELOG-3.5). Reference this issue: https://github.com/etcd-io/etcd/issues/14058