[FR]: Make hibernated tab's grayscale filter brighter in dark mode

Description

The current grayscale filter used to differentiate hibernated and active tabs is useful but makes some icons hard to make out.

Who's implementing?

• I'm willing to implement this feature myself

The problem

I like how the hibernated tabs are differentiated now, but the hard grayscale filter makes some icons disproportionately dark and hard to make out, specifically the red in the YouTube icon becomes almost invisible in dark mode.

Possible solutions

I think the grayscale filter could be changed to something more like 50% saturation instead, so there's still some color left to make out icons with lots of red, or those with low lightness contrast but high color contrast. You could argue that the latter is more of an issue of poor icon design on the site's part, but that's the world we live in.

Additional context

Related PR: #2435