Remove kibana_dashboard_only_user role

[kobelb]

Starting in 7.6, these are deprecated per #25879 on the Elasticsearch side. However, we still want to make some additional changes to the UI to make this even more clear in #45045. Starting in 8.0, we'll want to remove these roles entirely.

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Update: We are keeping the kibana_user role since we don't have a great reason to drop this right away. We will still mark this as deprecated and advise users to take advantage of the kibana_admin user instead.

We will still drop the kibana_dashboard_only_user role in 8.0, however, since this feature is being removed entirely.

Tasks:

• Remove the dashboard_mode plugin from x-pack and all references to dashboard_only in the code base. [Dashboard] Remove Legacy Dashboard Only Mode #108103
• Remove kibana_dashboard_only_user role from Elasticsearch's set of reserved roles remove dashboard only reserved role elasticsearch#76507
• Add upgrade assistant warning if a native user has the kibana_dashboard_only_user role, or if a role mapping contains this role.
• Inspect advanced settings across all spaces to see if the "dashboard only mode role" has been customized, and warn via upgrade assistant if any of these roles are assigned to a native user, , or if a role mapping contains this role.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[jportner]

Related: #42853 (also deals with roles that need to be changed)

[azasypkin]

Handled in #110960 (with the exception of inspecting of the Advanced Settings #110960 (comment))