CI: Unpin syn and proc-macro2 versions as a new version

`v0.50.0` 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