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