Update build dependencies and edition

Use the 2021 Rust edition. To use current version of clap changes were
adapted.

src/posix/info.rs

@@ -1,9 +1,9 @@
 use anyhow::Result;
-use clap::Clap;
+use clap::Parser;
 use posixmq::PosixMq;
 
 /// Print information about an existing message queue
-#[derive(Clap, Debug)]
+#[derive(Debug, Parser)]
 pub struct Info {
     /// Name of the queue
     #[clap(value_name = "QUEUE")]