Consider updating actix-derive to use syn@2

[LikeLakers2]

Hi! I noticed that actix-derive depends on syn@1. Perhaps you could consider updating it to use syn@2?

Some things worth noting for anyone who intends to take on this suggestion:

• syn@2's MSRV is Rust 1.56, up from syn@1's Rust 1.31. I am unsure how this fits with actix-derive, however I do note that actix's MSRV is 1.57 (according to the repository README), which probably means this shouldn't be a problem?
• Updating to syn@2 may not be as simple as swapping the version number - there are a number of breaking changes in syn@2, listed at https://github.com/dtolnay/syn/releases/tag/2.0.0.

If you do consider to transition to syn@2, you have my thanks! :)

[LikeLakers2]

@robjtede Ah, sorry for saying this sort of late, but I wouldn't label this as a "good first issue". I misunderstood how many changes would need to be made, and how much of what equates to a new architecture would need to be understood - and this only occurred to me after someone pointed it out on the issue I left on the Bevy Engine repository. bevyengine/bevy#8282 (comment)

[robjtede]

done in #554