[8.16] [ResponseOps][Rules]Deprecate `ruleFormV2` flag. (#1…

…96507) (#196702)

# Backport

This will backport the following commits from `main` to `8.16`:
- [[ResponseOps][Rules]Deprecate `ruleFormV2` flag.
(#196507)](#196507)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT
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`ruleFormV2` flag. (#196507)\n\nFixes #196493\r\n\r\n##
Summary\r\n\r\nIn a previous PR the `ruleFormV2` feature flag was
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`ruleFormV2`
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`ruleFormV2` flag. (#196507)\n\nFixes #196493\r\n\r\n##
Summary\r\n\r\nIn a previous PR the `ruleFormV2` feature flag was
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`ruleFormV2` flag. (#196507)\n\nFixes #196493\r\n\r\n##
Summary\r\n\r\nIn a previous PR the `ruleFormV2` feature flag was
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Co-authored-by: Antonio <[email protected]>

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];