N-2 index settings validation yields misleading error

[DaveCTurner]

Elasticsearch 9.x admits indices created in 7.x as long as the indices were marked as readonly in version ≥8.18.0, as indicated by the private index.verified_read_only setting. However if this setting is missing then you get an error indicating that the index must be marked as read-only using the setting [index.blocks.write] set to [true] before upgrading. This is misleading in a couple of ways:

• applying the setting index.blocks.write: true is not sufficient, you have to use the put-block API PUT /${INDEX}/_block/write to get the hidden index.verified_read_only setting too.

• you get this message even if the index has index.blocks.write: true, if it was marked read-only in an older version and/or by applying the setting index.blocks.write: true without using the put-block API.

Instead, we should guide users towards using the write block API and help them understand that index.blocks.write: true is not sufficient.

Relates https://discuss.elastic.co/t/378853/
Relates #119753

[elasticsearchmachine]

Pinging @elastic/es-distributed-indexing (Team:Distributed Indexing)