V14/feature/login image (#16063)

* update the login image to the v14 variant

* switch out the default value of the "alternative" logo to the dark version since it fits better in this version

src/Umbraco.Core/Configuration/Models/ContentSettings.cs

@@ -211,8 +211,8 @@ public class ContentSettings
     ///     of a light background (e.g. in mobile resolutions).
     /// </summary>
     /// <remarks>This is the alternative version to the regular logo found at <see cref="LoginLogoImage"/>.</remarks>
-    [DefaultValue(StaticLoginLogoImageAlternative)]
-    public string LoginLogoImageAlternative { get; set; } = StaticLoginLogoImageAlternative;
+    [DefaultValue(StaticLoginLogoImage)]
+    public string LoginLogoImageAlternative { get; set; } = StaticLoginLogoImage;
 
     /// <summary>
     ///     Gets or sets a value indicating whether to hide the backoffice umbraco logo or not.