[ResponseOps][Rules]Deprecate ruleFormV2 flag. (elastic#196507)

Fixes elastic#196493

## Summary

In a previous PR the `ruleFormV2` feature flag was renamed when it
should have been deprecated. This PR fixes that.

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

@@ -24,6 +24,8 @@ export const allowedExperimentalValues = Object.freeze({
   isUsingRuleCreateFlyout: false,
 });
 
+const deprecatedExperimentalValues = new Set(['ruleFormV2']);
+
 type ExperimentalConfigKeys = Array<keyof ExperimentalFeatures>;
 type Mutable<T> = { -readonly [P in keyof T]: T[P] };
 
@@ -56,7 +58,10 @@ export const parseExperimentalConfigValue = (configValue: string[]): Experimenta
 };
 
 export const isValidExperimentalValue = (value: string): boolean => {
-  return allowedKeys.includes(value as keyof ExperimentalFeatures);
+  return (
+    allowedKeys.includes(value as keyof ExperimentalFeatures) ||
+    deprecatedExperimentalValues.has(value)
+  );
 };
 
 export const getExperimentalAllowedValues = (): string[] => [...allowedKeys];