Reduce mismatched-lifetime-syntaxes suggestions to MaybeIncorrect
`cargo fix` does not have a way of distinguishing a suggestion with
multiple spans which should all be applied from multiple suggestions
where only one should be applied (see rust-lang/rust#53934). `cargo fix` only works
with `MachineApplicable` suggestions, so downgrading the applicability
will stop `cargo` from suggesting the user run `cargo
fix`. rust-analyzer does work with `MaybeIncorrect`, so interactive
fixes are still available.
r? `@ehuss`
ced61092ac