Replace proc-macro-error with proc-macro-error2

[kanpov]

As per rust-lang/docs.rs#2595 :

• proc-macro-error is unmaintained with creator being unreachable
• syn 1.x being pulled in likely means duplication of syn

So if the conversion is easy, the affected utoipa-gen should be converted asap imo.

[juhaku]

utoipa-gen does not use proc-macro-error but instead it is probably coming from some inner dependency: Duplicate for #1028

[juhaku]

To elaborate this further, it did use it before but the latest releases does not use it. It is at least known to be used by the rust_decimal crate. But perhaps there are others as well.

You could use cargo tree --format "{p} {f}" to debug and try to find where exactly the proc-macro-error dependency is coming from.