message: attributes: use correct minimal version of `syn`
  This sets the minimal version of `syn` to `1.0.43` - the actual minimal
  version that is required to build `tracing-attributes`.
  See this build failure in Tokio:
  CC: tokio-rs/tokio#4513
diff: @@ -35,7 +35,7 @@ proc-macro = true
  [dependencies]
  proc-macro2 = "1"
  -syn = { version = "1", default-features = false, features = ["full", "parsing", "printing", "visit", "visit-mut", "clone-impls", "extra-traits", "proc-macro"] }
  +syn = { version = "1.0.43", default-features = false, features = ["full", "parsing", "printing", "visit", "visit-mut", "clone-impls", "extra-traits", "proc-macro"] }
  quote = "1"
  [dev-dependencies]
  