Downgraded clap version to be compatible with MSRV

Author: Andrew J Westlake

Cargo.toml

@@ -111,7 +111,7 @@ required-features = ["async-std-runtime", "testing"]
 
 [dependencies]
 async-channel = { version = "1.6", optional = true }
-clap = { version = "4.0.10", optional = true }
+clap = { version = "3.2", optional = true }
 futures = "0.3"
 inventory = { version = "0.3", optional = true }
 once_cell = "1.14"