Fix msrv tests

Author: Veetaha

scripts/test-msrv.sh

@@ -34,6 +34,7 @@ with_log cd bon
 
 step echo '[workspace]' >> Cargo.toml
 
+step cargo update --precise 0.21.3  -p darling
 step cargo update --precise 1.0.15  -p itoa
 step cargo update --precise 1.0.101 -p proc-macro2
 step cargo update --precise 1.0.40  -p quote