fix(posthog): mark region required for update_experiment

posthog_update_experiment was the only experiment operation missing
from the region field's required-condition list; the other three
(create/get/list) already require it. Region is unconditionally
visible on this block (no condition key gates it), so this was a
required-ness inconsistency rather than a functional bug, but users
updating an EU experiment should still be prompted to select the
region explicitly rather than relying on the default.

Author: waleedlatif1

apps/sim/blocks/blocks/posthog.ts

@@ -100,6 +100,7 @@ export const PostHogBlock: BlockConfig<PostHogResponse> = {
           'posthog_list_feature_flags',
           // Experiments
           'posthog_create_experiment',
+          'posthog_update_experiment',
           'posthog_get_experiment',
           'posthog_list_experiments',
           // Data Management