chore: update highlights for dashboard-nvim

[sjclayton]

Remove deprecated hl group DashboardCenter and fully support dashboard-nvim

Before:

After:

[mvllow]

I've been noodling this one for a little, wondering if a more neutral look would fit better.

Some opinions:

• I don't think the load time is important enough to be an accent colour (perhaps "muted")
• Similar with icons, at least to me they seem more aesthetic than identifying (perhaps "subtle")
• Pretty sure in which-key (at some point, at least) had "iris" for the keys themselves, so "iris" could be good for that here

[sjclayton]

Not sure what to do here, as no matter what I do, nothing looks right without using accents.

So for now I'm going to close this, I may revisit it at a later time, but as I'm also no longer using dashboard-nvim, it may not be for a while.

[mvllow]

No worries, thank you for updating us here. Feel free to add any modifications to our recipes in the wiki too—I know there are plenty of people who would enjoy a more colourful dashboard :)