Update snapshot docs for new system index defaults (elastic#120174)

With elastic/elasticsearch#79670, you can only include system indices in a snapshot using feature states. This updates some references to taking a snapshots of the `.kibana*` system indices.

Relates to elastic/elasticsearch#81226

docs/setup/upgrade.asciidoc

@@ -44,13 +44,20 @@ a|
 [[upgrade-before-you-begin]]
 === Before you begin
 
-WARNING: {kib} automatically runs upgrade migrations when required. To roll back to an earlier version in case of an upgrade failure, you **must** have a {ref}/snapshot-restore.html[backup snapshot] available. This snapshot must include the `kibana` feature state or all `kibana*` indices. For more information see <<upgrade-migrations, upgrade migrations>>.
+[WARNING]
+====
+{kib} automatically runs upgrade migrations when required. To roll back to an
+earlier version in case of an upgrade failure, you **must** have a
+{ref}/snapshot-restore.html[backup snapshot] that includes the `kibana` feature
+state. Snapshots include this feature state by default.
+
+For more information, refer to <<upgrade-migrations, upgrade migrations>>.
+====
 
 Before you upgrade {kib}:
 
 * Consult the <<breaking-changes,breaking changes>>.
-* {ref}/snapshots-take-snapshot.html[Take a snapshot] of your data. To roll back to an earlier version, the snapshot must include the `kibana` feature state or all `.kibana*` indices. 
-* Although not a requirement for rollbacks, we recommend taking a snapshot of all {kib} indices created by the plugins you use such as the `.reporting*` indices created by the reporting plugin.  
+* {ref}/snapshots-take-snapshot.html[Take a snapshot] of your data. To roll back to an earlier version, the snapshot must include the `kibana` feature state.
 * Before you upgrade production servers, test the upgrades in a dev environment.
 * See <<preventing-migration-failures, preventing migration failures>> for common reasons upgrades fail and how to prevent these.
 * If you are using custom plugins, check that a compatible version is

docs/setup/upgrade/upgrade-migrations.asciidoc

@@ -151,17 +151,18 @@ In order to rollback after a failed upgrade migration, the saved object indices
 [float]
 ===== Rollback by restoring a backup snapshot:
 
-1. Before proceeding, {ref}/snapshots-take-snapshot.html[take a snapshot] that contains the `kibana` feature state or all `.kibana*` indices.
+1. Before proceeding, {ref}/snapshots-take-snapshot.html[take a snapshot] that contains the `kibana` feature state.
+   Snapshots include this feature state by default.
 2. Shutdown all {kib} instances to be 100% sure that there are no instances currently performing a migration.
 3. Delete all saved object indices with `DELETE /.kibana*`
-4. {ref}/snapshots-restore-snapshot.html[Restore] the `kibana` feature state or all `.kibana* indices and their aliases from the snapshot.
+4. {ref}/snapshots-restore-snapshot.html[Restore] the `kibana` feature state from the snapshot.
 5. Start up all {kib} instances on the older version you wish to rollback to.
 
 [float]
 ===== (Not recommended) Rollback without a backup snapshot:
 
 1. Shutdown all {kib} instances to be 100% sure that there are no {kib} instances currently performing a migration.
-2. {ref}/snapshots-take-snapshot.html[Take a snapshot] that includes the `kibana` feature state or all `.kibana*` indices.
+2. {ref}/snapshots-take-snapshot.html[Take a snapshot] that includes the `kibana` feature state. Snapshots include this feature state by default.
 3. Delete the version specific indices created by the failed upgrade migration. E.g. if you wish to rollback from a failed upgrade to v7.12.0 `DELETE /.kibana_7.12.0_*,.kibana_task_manager_7.12.0_*`
 4. Inspect the output of `GET /_cat/aliases`. If either the `.kibana` and/or `.kibana_task_manager` alias is missing, these will have to be created manually. Find the latest index from the output of `GET /_cat/indices` and create the missing alias to point to the latest index. E.g. if the `.kibana` alias was missing and the latest index is `.kibana_3` create a new alias with `POST /.kibana_3/_aliases/.kibana`.
 5. Remove the write block from the rollback indices. `PUT /.kibana,.kibana_task_manager/_settings {"index.blocks.write": false}`