[Discusss] Plugins should use system indices instead of dot indices

[gbbafna]

Describe the bug

ES introduced concept to system indices to supercede dot indices in ES version 7 . There are following benefits to use system indices :

1. Dedicated read and write pool
2. Prioritized recovery for system indices
3. Enable automated upgrade of indices .
4. There are some dot indices that do not necessarily fit the mold of a normal system index; instead they store data that the system produces with the intent that users can also query against this data. This has mingled with system index definition today , which should rather move to hidden index.
5. Ease of security configuration for system indices.

The dot indices created by user is a warning in 7.x and deprecated in 8.x in favor of system indices.

I think OpenSearch also move to system indices and move away from dot indices to get the benefits of same .

Additional context

1. System Indices elastic/elasticsearch#50251
2. Change AD indices to be hidden indices instead of system indices. opendistro-for-elasticsearch/anomaly-detection#394
3. Making shard copy count a multiple of attribute count #3462

[dblock]

I think we'd be open to making a similar change in OpenSearch (without looking at any non-APLv2 code).