Components.WebView: Fix CI failure building with WebKitGTK >= 2.31

The `webkit_settings_set_enable_plugins` method has been deprecated in
2.31, and calling it emits a warning which trips up the test suite.

src/client/components/components-web-view.vala

@@ -325,7 +325,9 @@ public abstract class Components.WebView : WebKit.WebView, Geary.BaseInterface {
         setts.enable_media_stream = false;
         setts.enable_offline_web_application_cache = false;
         setts.enable_page_cache = false;
+#if WEBKIT_PLUGINS_SUPPORTED
         setts.enable_plugins = false;
+#endif
         setts.hardware_acceleration_policy =
             WebKit.HardwareAccelerationPolicy.NEVER;
         setts.javascript_can_access_clipboard = true;

src/client/meson.build

@@ -192,6 +192,10 @@ client_vala_args += [
   )
 ]
 
+if webkit2gtk.version().version_compare('<2.31')
+  client_vala_args += [ '--define=WEBKIT_PLUGINS_SUPPORTED' ]
+endif
+
 # Main client application library
 client_lib = shared_library(
   client_package,