btm: use named colours for rainbow config

Removed orange and brown to make a 6 colour rainbow, looks better on
my 6-core processor

home/bottom/default.nix

@@ -2,7 +2,7 @@
   programs.bottom = {
     enable = true;
     settings.colors = with config.lib.stylix.colors.withHashtag; let
-      rainbow = [base08 base09 base0A base0B base0C base0D base0E base0F];
+      rainbow = [red yellow green cyan blue magenta];
     in {
       table_header_color = base05;
       all_cpu_color = base05;