patch 8.2.0906: when setting 'termguicolors' SpellBad is no longer red

Problem:    When setting 'termguicolors' SpellBad is no longer red.
Solution:   Only use the RGB guisp color for cterm when using the "underline"
            or "undercurl" attributes to avoid the background color to be
            cleared. Also make t_8u empty when the termresponse indicates a
            real xterm. (closes #6207)

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    906,
 /**/
     905,
 /**/