Remove the link to the Q4 survey from the banner (#12589)

They survey is closing today. Is there something else that we'd like to
put in the banner instead?

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---------

Co-authored-by: Parker Lougheed <[email protected]>

Author: zanderso

src/_includes/banner.html

@@ -1 +1 @@
-Help improve Flutter! <a href="https://google.qualtrics.com/jfe/form/SV_5ulaitlgYw3Xps2?Source=Website">Take our Q4 survey</a><br>
+Build high quality, feature-rich apps with the new Flutter Extension for Gemini CLI. <a href="https://blog.flutter.dev/meet-the-flutter-extension-for-gemini-cli-f8be3643eaad" target="_blank">Learn more</a><br>