Update to sqlx 0.8.0 (#794)

This updates sqlx to the latest version (0.8.0), and also updates
rusqlite to 0.31.0. The latest version of rusqlite is 0.32.0, but we
can't use that at the same time as sqlite, because [it depends on
libsqlite3-sys
0.30.0](https://crates.io/crates/rusqlite/0.32.0/dependencies), which
conflicts with the version [sqlx 0.8.0 depends
on](https://crates.io/crates/sqlx-sqlite/0.8.0/dependencies) (0.28.0).
This is the same version that [rustqlite 0.31.0
uses](https://crates.io/crates/rusqlite/0.31.0/dependencies), so we use
that version instead.

Related upstream issue:
launchbadge/sqlx#3295