fix: env variants event changelog (#4712)

https://linear.app/unleash/issue/2-1397/fix-environment-variants-change-event-does-not-include-changelogs

By running `applyPatch` without cloning the `oldVariants`, `applyPatch`
would patch the `oldVariants` by reference, effectively making them the
same as the `newVariants`. This fix `deepClone`s the oldVariants when
sending them in as an `applyPatch` parameter so that the `oldVariants`
variable is left untouched.


![image](https://github.com/Unleash/unleash/assets/14320932/089e118d-c5c4-432e-b11c-08d362ce155d)

src/lib/services/feature-toggle-service.ts

@@ -1898,7 +1898,10 @@ class FeatureToggleService {
             featureName,
             environment,
         );
-        const { newDocument } = await applyPatch(oldVariants, newVariants);
+        const { newDocument } = await applyPatch(
+            deepClone(oldVariants),
+            newVariants,
+        );
         return this.crProtectedSaveVariantsOnEnv(
             project,
             featureName,