ruff_db: simplify lifetimes on `DiagnosticDisplay`
I initially split the lifetime out into three distinct lifetimes on
near-instinct because I moved the struct into the public API. But
because they are all shared borrows, and because there are no other APIs
on `DisplayDiagnostic` to access individual fields (and probably never
will be), it's probably fine to just specify one lifetime. Because of
subtyping, the one lifetime will be the shorter of the three.
There's also the point that `ruff_db` isn't _really_ a public API, since
it isn't a library that others depend on. So my instinct is probably a
bit off there.
