marionette: Capabilities::platform_version optional

Firefox 107 does not include the `platform_version` member in the object
in the `capabilities` member of the response to the `NewSession`
command.

src/marionette/message.rs

@@ -41,7 +41,7 @@ pub struct Capabilities {
     pub webdriver_click: bool,
     pub page_load_strategy: String,
     pub platform_name: String,
-    pub platform_version: String,
+    pub platform_version: Option<String>,
     // proxy:
     pub set_window_rect: bool,
     pub timeouts: Timeouts,