Revert "Set minimum rust version in Cargo.toml"

Cargo.toml

@@ -10,7 +10,6 @@ license = "BSD-3-Clause"
 keywords = ["prolog", "prolog-interpreter", "prolog-system"]
 categories = ["command-line-utilities"]
 build = "build.rs"
-rust-version = "1.57"
 
 [workspace]
 members = ["crates/num-rug-adapter",
@@ -73,4 +72,4 @@ predicates-core = "1.0.2"
 serial_test = "0.5.1"
 
 [profile.release]
-debug = true
+debug = true