Dep: use fixed version 1.0.79 for proc-macro2

proc-macro2 1.0.80 produces an error:

```text
Error:    --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.80/src/wrapper.rs:871:21
    |
871 |                     proc_macro::Literal::byte_character(byte)
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #115268 <rust-lang/rust#115268> for more information
    = help: add `#![feature(proc_macro_byte_character)]` to the crate attributes to enable
    = note: this compiler was built on 2024-03-19; consider upgrading it if it is out of date
```

Cargo.toml

@@ -27,7 +27,7 @@ futures = "0.3"
 lazy_static = "1.4.0"
 maplit = "1.0.2"
 pretty_assertions = "1.0.0"
-proc-macro2 = "1.0"
+proc-macro2 = "=1.0.79"
 quote = "1.0"
 rand = "0.8"
 semver = "1.0.14"