Add a warning to the media caption composer. (#3574)

We can now remove the feature flag.

ElementX/Sources/Application/AppSettings.swift

@@ -240,6 +240,10 @@ final class AppSettings {
     
     @UserPreference(key: UserDefaultsKeys.optimizeMediaUploads, defaultValue: true, storageType: .userDefaults(store))
     var optimizeMediaUploads
+    
+    /// Whether or not to show a warning on the media caption composer so the user knows
+    /// that captions might not be visible to users who are using other Matrix clients.
+    let shouldShowMediaCaptionWarning = true
 
     // MARK: - Element Call