fix(deps): update rust crate darling to 0.21.0 (#104)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

spec/Cargo.toml

@@ -9,6 +9,6 @@ path = "src/lib.rs"
 proc-macro = true
 
 [dependencies]
-darling = "0.20.8"
+darling = "0.21.0"
 syn = "2.0.103" # needed for `darling`