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. Cherry picked from !624 commit 28921890
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. Cherry picked from !624 commit 28921890