patch 9.1.0068: Visual highlighting can still be improved

Problem:  Visual highlighting can still be improved
Solution: Update Visual highlighting for 8 color terminals,
          use uniform grey highlighting for dark and light bg
          (Maxim Kim)

Update terminal Visual

1. Use `ctermbg=Grey ctermfg=Black` for both dark and light

This uniforms Visual highlighting between default dark and light colors
And should work for vim usually detecting light background for terminals
with black/dark background colors.

Previously used `ctermfg=White` leaks `cterm=bold` if available colors
are less than 16.

2. Use `term=reverse cterm=reverse ctermbg=NONE ctermfg=NONE`
   for terminals reporting less than 8 colors available

If the terminal has less than 8 colors, grey just doesn't work right

closes: #13940

Signed-off-by: Maxim Kim <habamax@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/dumps/Test_echowin_showmode.dump b/src/testdir/dumps/Test_echowin_showmode.dump
index ec51102..9a73bbf 100644
--- a/src/testdir/dumps/Test_echowin_showmode.dump
+++ b/src/testdir/dumps/Test_echowin_showmode.dump
@@ -1,4 +1,4 @@
->o+0&#ffffff0|n+0#ffffff16#6c6c6c255|e| | +0#0000000#ffffff0@70
+>o+0&#ffffff0|n+0#0000001#a8a8a8255|e| | +0#0000000#ffffff0@70
 |t|w|o| @71
 |~+0#4040ff13&| @73
 |~| @73