help: Update links and examples to use zulip-flutter repo.

The one remaining link is for the instructions to download the
legacy app APK file for Android.

Part of #35643.

help/add-a-custom-linkifier.md

@@ -101,10 +101,10 @@ variants. For example, the Zulip development community
 
 {start_tabs}
 
-* Pattern: `#M(?P<id>[0-9]+)`
-* URL template: `https://github.com/zulip/zulip-mobile/issues/{id}`
-* Original text: `#M2468`
-* Automatically links to: `https://github.com/zulip/zulip-mobile/issues/2468`
+* Pattern: `#F(?P<id>[0-9]+)`
+* URL template: `https://github.com/zulip/zulip-flutter/issues/{id}`
+* Original text: `#F245`
+* Automatically links to: `https://github.com/zulip/zulip-flutter/issues/245`
 
 {end_tabs}
 

help/mobile-app-install-guide.md

@@ -71,8 +71,8 @@ not be updated with bug fixes or new features.
 
 1. Uninstall the Zulip app, if it's already installed.
 
-1. Download [the legacy
-   app](https://github.com/zulip/zulip-mobile/releases/latest) APK file.
+1. Download [the legacy app](https://github.com/zulip/zulip-mobile/releases/latest)
+   APK file.
 
 1. Open the APK file, and follow prompts to install it.
 

help/support-zulip-project.md

@@ -40,7 +40,7 @@ Collective](https://opencollective.com/zulip).
   practices.
 * **Star** Zulip on GitHub. There are four main repositories:
   [server/web](https://github.com/zulip/zulip),
-  [mobile](https://github.com/zulip/zulip-mobile),
+  [mobile](https://github.com/zulip/zulip-flutter),
   [desktop](https://github.com/zulip/zulip-desktop), and
   [Python API](https://github.com/zulip/python-zulip-api).
 * **Review** Zulip on product comparison websites, such as