a11y: Improve color contrast for help button

[t-hamano]

The text color for the help button is $gray-600, which meets accessibility requirements for large text, but not for normal text. For normal text, we should use a color at least darker than $gray-700.

Or, we could just leave it as the normal text color.

Before ($gray-600, #949494)	After ($gray-700, #757575)

[t-hamano]

The svg element in the button component is applied with currentColor by default.

https://github.com/WordPress/gutenberg/blob/4d1aa40f02c6a46e08b1496f74c66a059611b0a2/packages/components/src/button/style.scss#L345

[mikachan]

Thanks @t-hamano! I think using $gray-700 is a good change and seems like a suitable contrast for this font size.