core: Add version 23 to XCF switch statement

Resolves #13124
In 3f3b29ba we bumped XCF version to 23 due to
new blending spaces. However, we did not update
the version check switch statement to accept this
version. This patch adds a new case to handle version
23.

app/core/gimpimage.c

@@ -3187,6 +3187,7 @@ gimp_image_get_xcf_version (GimpImage    *image,
     case 20:
     case 21:
     case 22:
+    case 23:
       if (gimp_version)   *gimp_version   = 300;
       if (version_string) *version_string = "GIMP 3.0";
       break;