Upgrade proc-macro2

Our CI's Nightly Checks are failing with 'unknown feature
`proc_macro_span_shrink`' error. This problem is explained in
rust-lang/rust#113152.

As mentioned in
rust-lang/rust#113152 (comment)
the issue may be solved by updating proc-macro2 to version 1.0.60+

Run 'cargo update --package proc-macro2' to fix the issue.

Signed-off-by: Tomás González <[email protected]>

Author: tgonzalezorlandoarm