Specify shared crate metadata at the workspace level

[mkrasnitski]

This keeps crate metadata in sync across the whole workspace, including MSRV and edition, plus adds some missing metadata fields to serenity's subcrates.

Feedback wanted: should the authors field also be applied to the whole workspace? Note that workspace fields are opt-in via field.workspace = true and can be overridden by simply providing a field = ... value.

[arqunis]

Feedback wanted: should the authors field also be applied to the whole workspace? Note that workspace fields are opt-in via field.workspace = true and can be overridden by simply providing a field = ... value.

I believe that should be set individually per crate, especially since we want to integrate poise into the Serenity repository, which has a different authors field.