update package edition to 2018 for the log crate's MSRV check.

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "structured-logger"
 version = "0.4.1"
-edition = "2021"
+edition = "2018"
 description   = """
 A logging implementation for the log crate that logs structured values either synchronous or asynchronous, as JSON, CBOR, or any other format, into a file, stderr, stdout, or any other destination.
 """

src/lib.rs

@@ -191,7 +191,7 @@ impl Builder {
     /// you want to handle the error yourself.
     pub fn init(self) {
         self.try_init()
-            .unwrap_or_else(|err| panic!("failed to initialize the logger: {err}"));
+            .unwrap_or_else(|err| panic!("failed to initialize the logger: {}", err));
     }
 
     /// Try to initialize the logger.