Update hex value for secondary text to improve contrast

Fixes in this PR

The secondary text colour value has been updated to improve the
contrast of components that use the ```govuk-colour("dark-grey")```
sass variable. The value has been updated from ```#6f777b``` to ```#626a6e```

This is a small incremental change that improves the contrast ratio from 4.56:1 to 5.51:1.

Fixes #1134

src/govuk/settings/_colours-palette.scss

@@ -70,7 +70,7 @@ $_govuk-colour-palette-modern: (
   "purple": #4c2c92,
 
   "black": #0b0c0c,
-  "dark-grey": #6f777b,
+  "dark-grey": #626a6e,
   "mid-grey": #b1b4b6,
   "light-grey": #f3f2f1,
   "white": #ffffff,