Bump proc-macro2 from 1.0.50 to 1.0.66 in /src/bare-metal/microcontro… (

#1160)

…llers/examples

The proc-macro2 dependency at 1.0.50 results in a build error mentioning
an unknown feature `proc_macro_span_shrink` that no longer exists in
nightly. Per rust-lang/rust#113152, updating
proc-macro2 to the latest version fixes the error.

The build error I saw before updating the proc-macro2 dependency was the
following:

```
> cargo build --bin minimal
   Compiling proc-macro2 v1.0.50
   Compiling bare-metal v0.2.5
   Compiling typenum v1.16.0
   Compiling nrf52833-hal v0.14.1
error[E0635]: unknown feature `proc_macro_span_shrink`
  --> /home/chcl/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.50/src/lib.rs:92:30
   |
92 |     feature(proc_macro_span, proc_macro_span_shrink)
   |                              ^^^^^^^^^^^^^^^^^^^^^^

   Compiling cortex-m v0.7.7
For more information about this error, try `rustc --explain E0635`.
error: could not compile `proc-macro2` (lib) due to previous error
warning: build failed, waiting for other jobs to finish...
```