The latest version of `rental` (v0.5.6) contains a fix that allows it to
compile without relying on the pretty-print back-compat hack.

Hopefully, there are no longer any crates relying on the affected
versions of the (much less popular) `procedural-masquerade` crate. This
should allow us to target the pretty-print back-compat hack specifically
to older versions of `rental`, and specifically mention upgrading to
`rental` v0.5.6 in the lint message.

This updates the 'cssparser' and 'procedural-masquerade' deps
to versions that no longer depend on the proc-macro back-compat
hack.