use weak dependancy feature for `static` when msrv reaches 1.60

#302 (closed) was fixed by having multiple feature flags which enable static compilation for different dependencies.
With the new rust-1.60, there is a syntax for doing the same thing with a single feature flag.

When Arti up its msrv to 1.60 (which will probably be in quiet some time), we should take advantage of this new feature.