Deprecate the setting plugins.security.system_indices.indices for removal in OpenSearch 4

[cwperks]

Since OpenSearch 2.17, the security plugin has been conscious of the SystemIndexRegistry in OpenSearch core which is composed of all of the indices declared across all SystemIndexPlugins.

A plugin utilizing a system index will extend SystemIndexPlugin and declare system indices with SystemIndexPlugin.getSystemIndexDescriptors()

I propose deprecating plugins.security.system_indices.indices to discourage any plugin from further adding to the manually tracked list here now that we are able to get the registry programatically.

[shikharj05]

nit: should we create a label and tag all 4.0 items with the label to filter during 4.0 work?

[nibix]

In the context of my RFC on the "protected indices" feature #5598 I am wondering whether system indices might be also useful for purposes other than plugins.

In the end, it is an additional protection mechanism for indices. Plugins have a special way to gain access to these indices, but there's also the system index permission, which gives any user access to these indices.

Thus, it could be also used like the "protected indices" feature with the difference that it uses permissions instead of enumerated role names (which is an advantage IMHO).

[nibix]

Related: #5619

[cwperks]

nit: should we create a label and tag all 4.0 items with the label to filter during 4.0 work?

Created the v4.0.0 label and added it to this issue.