Update the supported versions for archive indices (#3785)

Fixes #3577

The content of [archive
functionality](https://www.elastic.co/docs/deploy-manage/upgrade/deployment-or-cluster/reading-indices-from-older-elasticsearch-versions)
needs to be updated to include also version 7, as it already does in the
[preparation
steps](https://www.elastic.co/docs/deploy-manage/upgrade/prepare-to-upgrade#run-the-upgrade-assistant)
for an upgrade.

Author: yetanothertw

deploy-manage/upgrade/deployment-or-cluster/reading-indices-from-older-elasticsearch-versions.md

@@ -9,7 +9,7 @@ products:
 
 # Reading indices from older {{es}} versions [archive-indices]
 
-{{es}} has full query and write support for indices created in the previous major version. If you have indices created in {{es}} versions 5 or 6, you can use the archive functionality to import them into newer {{es}} versions as well.
+{{es}} has full query and write support for indices created in the previous major version. If you have indices created in {{es}} versions 5, 6, or 7, you can use the archive functionality to import them into newer {{es}} versions as well.
 
 The archive functionality provides slower read-only access to older {{es}} data, for compliance or regulatory reasons, the occasional lookback or investigation, or to rehydrate parts of it. Access to the data is expected to be infrequent, and can therefore happen with limited performance and query capabilities.
 
@@ -50,7 +50,7 @@ Due to `_source` access, the data can also be [reindexed](https://www.elastic.co
 
 ## Upgrade older {{es}} clusters [_how_to_upgrade_older_es_clusters]
 
-To upgrade older {{es}} 5 or 6 clusters: 
+To upgrade older {{es}} 5, 6, or 7 clusters: 
 
 1. Take a snapshot of the indices in the old cluster. 
 2. Delete any indices created before 8.0.0.