Migration docs: remove content related to 6.x versions

docs/setup/upgrade/upgrade-migrations.asciidoc

@@ -32,7 +32,6 @@ adds a write block to the outdated index. Table 1 lists the saved objects indice
 [options="header"]
 |=======================
 |Upgrading from version | Outdated index (alias)
-| 6.0.0 through 6.4.x    | `.kibana`
 
 `.kibana_task_manager_7.12.0_001` (`.kibana_task_manager` alias)
 | 6.5.0 through 7.3.x    | `.kibana_N` (`.kibana` alias)
@@ -145,9 +144,6 @@ Matching index templates that specify `settings.refresh_interval` or `mappings`
 
 To make sure the index templates won't apply to new `.kibana*` indices, narrow down the {data-sources} of any user-defined index templates.
 
-NOTE: In {kib} 6.5.0 and earlier, {kib} creates a `kibana_index_template:.kibana` index template
-and uses a `.kibana` index pattern. You do not need to change or remove the index template.
-
 [float]
 ===== An unhealthy {es} cluster
 Problems with your {es} cluster can prevent {kib} upgrades from succeeding. Ensure that your cluster has: