docs(flutter): recommend SDK 5.14.0 for latest survey features (#14994)

* docs(flutter): recommend SDK 5.14.0 for latest survey features

Updates the Flutter surveys documentation to recommend version 5.14.0 or
later for the latest survey customization features, while keeping 5.8.0
as the minimum requirement.

Slack thread: https://posthog.slack.com/archives/C07QD3LT8U9/p1770911211150029?thread_ts=1770902761.416349&cid=C07QD3LT8U9

https://claude.ai/code/session_0177JmBPTnUhgxVP71hriERG

* Apply suggestion from @adboio

* Apply suggestion from @adboio

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: adboio

contents/docs/surveys/installation/flutter.mdx

@@ -11,7 +11,7 @@ import FlutterInstallManual from "../../integrate/_snippets/install-flutter-manu
 
 <Step title="Add PostHog to your app" badge="required">
 
-Using it requires PostHog's Flutter SDK version >= 5.8.0, but it's recommended to always use [the latest version](/docs/sdk-doctor).
+Using it requires PostHog's Flutter SDK version >= 5.8.0, but version 5.14.0 or later is recommended for the latest survey customization features. You can check your current version in PostHog using the [SDK doctor](/docs/sdk-doctor).
 
 > **Note:** For mobile surveys, you must setup the SDK manually.
 
@@ -103,7 +103,7 @@ config.surveys = false; // Disable surveys
 - On Android, requires API >= 26.
 - On iOS, minimum deployment target is [iOS13](/docs/libraries/ios).
 - On Web, surveys are powered by the [JavaScript Web SDK](/docs/surveys/installation/web).
-- Requires PostHog Flutter SDK version >= 5.8.0
+- Requires PostHog Flutter SDK version >= 5.8.0 (5.14.0+ recommended for latest features)
 
 ## Troubleshooting