riscv-rt-macros: update syn to v2.0

[agarof]

Upgrade syn in order to deduplicate dependencies in projects depending on riscv-rt, as most other crates have upgraded to version 2.
Tested by building and flashing a simple hello world to the ESP32-C3, with no difference between the two versions.

While syn's README states Version requirement: Syn supports rustc 1.61 and up., riscv-rt-macros still builds with 1.59.

[romancardenas]

Thanks for the PR! It looks like we need to bump the MSRV to 1.61
@agarof please also remember to update the CI workflow so it passes

[agarof]

I bumped the MSRV of riscv-rt to 1.61, CI should pass now

[almindor]

Thank you!

[almindor]

I think this might be good as is nevertheless we should also bump the CI rust versions at https://github.com/rust-embedded/riscv/blob/master/.github/workflows/riscv-rt.yaml#L13-L14

[agarof]

The change has already been done here : https://github.com/rust-embedded/riscv/pull/221/files#diff-0ec8e1c75aebc15aec4ac8becbc3c0aa0e7980346b90cf55ba03dedde8630911R14