Dark mode link hover: increase brightness instead lower opacity.

[isoos]

• Opacity change on hover may not be a good idea #8493
• Note: filter: brightness() did not work for me on images, but maybe other factors are at play.
• I'm not yet sure how to resolve the same opacity issue in light mode, given we are using it the current way for a long time, and also the brightness issue above, so I'm limiting this change to dark mode for now, as it seems to be an improvement there.

[isoos]

I've started a staging push with this, so that we can take a look at it at least in dark mode, and let's discuss how we should try to solve it for images too.

/cc @parlough

[jonasfj]

do we have opacity other than 1?

[isoos]

Yes, the version tab icons have it on 0.7 by default (but uneffected as they are on the element). However, in this case this is to counter the main rule here:
https://github.com/dart-lang/pub-dev/blob/master/pkg/web_css/lib/src/_base.scss#L98-L100

I shall move that out to a separate light-theme/dark-theme styles so we don't need the override.

[isoos]

Done, PTAL.