Quick-fix Users can’t upgrade their clusters when riskScore experim…

…ental feature is enabled (elastic#146780)

Users can't upgrade their clusters when an old experimental feature is
enabled.

While we don't have a final solution for this issue, I am adding the
experimental flag configuration back to prevent users from having this
frustrating experience.

Quick-fix for: elastic#146777
Original report:
https://elastic.slack.com/archives/C6E3MTCD7/p1669236299374339

Fix
```
FATAL Error: [config validation of [xpack.securitySolution].enableExperimental]: [riskyUsersEnabled] is not allowed. Allowed values are: tGridEnabled, tGridEventRenderedViewEnabled, excludePoliciesInFilterEnabled, kubernetesEnabled, disableIsolationUIPendingStatuses, pendingActionResponsesWithAck, policyListEnabled, policyResponseInFleetEnabled, previewTelemetryUrlEnabled, responseActionsConsoleEnabled, insightsRelatedAlertsByProcessAncestry, extendedRuleExecutionLoggingEnabled, socTrendsEnabled, responseActionsEnabled
```

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

x-pack/plugins/security_solution/common/experimental_features.ts

@@ -80,6 +80,16 @@ export const allowedExperimentalValues = Object.freeze({
    * Enables the `get-file` endpoint response action
    */
   responseActionGetFileEnabled: false,
+
+  /**
+   * Keep DEPRECATED experimental flags that are documented to prevent failed upgrades.
+   * https://www.elastic.co/guide/en/security/current/user-risk-score.html
+   * https://www.elastic.co/guide/en/security/current/host-risk-score.html
+   *
+   * Issue: https://github.com/elastic/kibana/issues/146777
+   */
+  riskyHostsEnabled: false, // DEPRECATED
+  riskyUsersEnabled: false, // DEPRECATED
 });
 
 type ExperimentalConfigKeys = Array<keyof ExperimentalFeatures>;