Merge pull request #420 from moka-rs/kani-2024-04-19/v0.12

CI: Unpin `syn` and `proc-macro2` versions as a new version `0.50.0` of `kani` is released

.ci_extras/pin-crate-vers-kani.sh

@@ -4,5 +4,4 @@ set -eux
 
 # Pin some dependencies to specific versions for the nightly toolchain
 # used by Kani verifier.
-cargo update -p [email protected] --precise 2.0.58
-cargo update -p proc-macro2 --precise 1.0.79
+# cargo update -p crate-name --precise x.y.z