[Reporting] Back off on the approach to remove support for the `roles.enabled` setting.

[tsullivan]

Kibana version: 8.0 and the latest 7.x

Removal of supporting roles.enabled is too big of a change to force into 8.0. The removal should be phased more gradually, using telemetry data for acceptance criteria.

Steps to reproduce:

1. Start Kibana with reporting config at the defaults
2. View a warning that roles.enabled support is going away in 8.0: https://github.com/elastic/kibana/pull/100427/files#r714993527

Expected behavior:

Granting reporting privilege through a "reporting_user" role will not be supported starting in 8.0 in a future version.

[elasticmachine]

Pinging @elastic/kibana-reporting-services (Team:Reporting Services)

[tsullivan]

Closed via #114950