[meta] fix up some schema warnings from tombi

[sunshowers]

I recently set up Tombi in my IDE, and it produced a few warnings. Clean up the easy ones: nextest tables out of order, and the deprecated authors field in Cargo.toml.

There are a few harder ones left which I haven't touched:

• out-of-order tables in nexus/examples/
• out-of-order dotted keys in package-manifest.toml
• rustfmt.toml being rejected (which actually seems like an outdated schema)

(I haven't set up CI to do Tombi validation so this will probably regress in the future. Happy to do that if there's interest!)

[sunshowers]

This field is now deprecated.

[sunshowers]

@iliana would love your thoughts here on whether toml validation is worth setting up in CI

[iliana]

Can we make it enforce sorting the dependencies sections? If so absolutely yes.

cargo add only attempts to sort new dependencies if packages are already sorted and most aren't. I try to fix things as I work on them but it's really easy for them to fall out of line.