Increase the contrast of the selected block outline

[afercia]

Splitting this out from #11737

Indication of the active part in an UI is of fundamental importance for keyboard users and benefits all users. Usually this applied to the element that has focus. In Gutenberg it applies also to the "selected" block.

According to the WCAG 2.1, non-text contrast (which means contrast of UI components) must be at least 3:1. In #11737 (comment) it was agreed the selected outline contrast can be increased slightly.

The current "selected" indication is a very light grey outline:

Among the current available greys in the color palette, #8d96a0 is the one to use for non-text contrast against a white background. However, this grey needs to be slightly adjusted. Just checked and, though it's a matter of roundings, the actual contrast ratio against white is 2.99:1 🙂 For formal compliance it must be 3:1.

[afercia]

Well, actually some tools report a 2.99 ratio, other tools 3.00:

3.00
https://jdlsn.com/color/?type=hex&color=8d96a0&color2=ffffff

2.99
https://contrast-ratio.com/#%238d96a0-on-%23ffffff

I'd suggest to slightly adjust it anyways, just in case.

[mtias]

If the threshold is so small, makes sense to update it slightly to be squarely within it.

[noisysocks]

Anyone able to pick a colour for us to use here?

[timwright12]

@afercia I think this might be a dupe I was working on it in this PR last week #12478

[afercia]

Oh yes this is a duplicate of #12254 (or the other way around).