Add a flag showing when reindex is required for indices/data streams returned by /_migration/deprecations

[jloleysens]

Response from /_migration/deprecations when indices/data streams need to be reindexed:

// indices
    "myindex": [
      {
        "level": "critical",
        "message": "Old index with a compatibility version < 8.0",
        "url": "https://www.elastic.co/guide/en/elasticsearch/reference/master/breaking-changes-9.0.html",
        "details": "This index has version: 7.17.25",
        "resolve_during_rolling_upgrade": false
      }
    ]
// data streams
    "my-data-stream": [
      {
        "level": "critical",
        "message": "Old data stream with a compatibility version < 8.0",
        "url": "https://www.elastic.co/guide/en/elasticsearch/reference/master/breaking-changes-9.0.html",
        "details": "This data stream has backing indices that were created before Elasticsearch 8.0.0",
        "resolve_during_rolling_upgrade": false,
        "_meta": {
          "backing_indices": {
            "need_upgrading": {
              "count": 1,
              "searchable_snapshots": {
                "count": 0,
                "fully_mounted": {
                  "count": 0
                },
                "partially_mounted": {
                  "count": 0
                }
              }
            },
            "count": 1
          }
        }
      }
    ]

For data streams programs can check _meta to detect whether reindexing is required, but for indices we only have a hint in the user-facing message: "Old index with a compatibility version < 8.0". Kibana currently inspects this information to determine if we need to ask users to reindex. To show users something like:

It would be great if we could return _meta: { reindex_required: true|false } for both data streams and indices cases for consistency and clarity.

Related elastic/kibana#202669

[elasticsearchmachine]

Pinging @elastic/es-data-management (Team:Data Management)