[Breaking change] Move xpack.telemetry.* config to telemetry.*

[afharo]

Change description

Which release will ship the breaking change?

8.0

Describe the change. How will it manifest to users?

The configuration parameters prefixed by xpack.telemetry in the kibana.yml file will make the config validation to be rejected and Kibana will crash.

In the logs, it will show:

 FATAL  Error: Unknown configuration key(s): "xpack.telemetry.enabled". Check for spelling errors and ensure that expected plugins are installed.

How many users will be affected?

The deprecation warning has been there ever since 7.5. And users don't tend to mess up with the telemetry configurations (unless they are disabling it). We would expect a small percentage of our user base to be affected by this breaking change.

What can users do to address the change manually?

Users can remove the prefix xpack. for every xpack.telemetry entry in the configuration file.

How could we make migration easier with the Upgrade Assistant?

If the Upgrade Assistant has access to the raw config file (or config parameters), try to identify any entries prefixed with xpack.telemetry.

Are there any edge cases?

Not that I'm aware of.

Test Data

Run Kibana with the config entry xpack.telemetry.enabled: false. It doesn't fail on 7.x but it crashes on master with the error log shown below:

 FATAL  Error: Unknown configuration key(s): "xpack.telemetry.enabled". Check for spelling errors and ensure that expected plugins are installed.

Cross links

The changes were introduced in this PR: #51142

[alisonelizabeth]

I'm going to remove the Elasticsearch UI team label and add the Core team, as any deprecations should be handled via the core deprecations service (#97159).

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[lukeelmers]

All of the above items are deprecated yml configs, which means they're already being surfaced by the deprecations service for use in the upgrade assistant. And the breaking change for this config has already been made in #51142

I'll go ahead and close this issue, but @cjcenizal feel free to re-open if it is needed for some other reason.

[lukeelmers]

Reopening; still need to remove the urls from the config file