Document a known issue about the v2 to v3 migration

* Update the 3.4 doc about the known issue of v2 to v3 migration
* Remove v2 migration doc from 3.5 and 3.6

Fixes kubernetes#8804
Signed-off-by: Hongbin Lu <[email protected]>

Author: hongbin

content/en/docs/v3.4/op-guide/v2-migration.md

@@ -40,6 +40,12 @@ First, all members in the etcd cluster must converge to the same state. This can
 
 Second, migrate the v2 keys into v3 with the [migrate][migrate_command] (`ETCDCTL_API=3 etcdctl migrate`) command. The migrate command writes keys in the v2 store to a user-provided transformer program and reads back transformed keys. It then writes transformed keys into the mvcc store. This usually takes at most tens of seconds.
 
+{{% alert title="known issue" color="warning" %}}
+  The migrate command has a known issue that new members added after the migration do not receive existing data if no .snap file exists.
+  The workaround is to perform a snapshot restore after the migration.
+  For more information see [GitHub issue](https://github.com/etcd-io/etcd/issues/8804).
+{{% /alert %}}
+
 Restart the etcd members and everything should just work.
 
 For etcd v3.3+, run `ETCDCTL_API=3 etcdctl endpoint hashkv --cluster` to ensure key-value stores are consistent post migration.

content/en/docs/v3.5/op-guide/v2-migration.md

@@ -6,10 +6,6 @@ description: Documentation list for upgrading etcd clusters and applications
 
 This section contains documents specific to upgrading etcd clusters and applications.
 
-## Moving from etcd API v2 to API v3
-
-* [Migrate applications from using API v2 to API v3][migrate-apps]
-
 ## Upgrading an etcd v3.x cluster
 * [Upgrade etcd from 3.0 to 3.1](../upgrade_3_1/)
 * [Upgrade etcd from 3.1 to 3.2](../upgrade_3_2/)

content/en/docs/v3.5/upgrades/upgrading-etcd.md

@@ -6,10 +6,6 @@ description: Documentation list for upgrading etcd clusters and applications
 
 This section contains documents specific to upgrading etcd clusters and applications.
 
-## Moving from etcd API v2 to API v3
-
-* [Migrate applications from using API v2 to API v3][migrate-apps]
-
 ## Upgrading an etcd v3.x cluster
 * [Upgrade etcd from 3.0 to 3.1](../upgrade_3_1/)
 * [Upgrade etcd from 3.1 to 3.2](../upgrade_3_2/)