replace tempdir with tempfile

`tempdir` has been abandoned, last published 6 years ago, and some of
its dependencies have known security issues.

The recommended replacement is the `tempfile` crate.

russh-keys/Cargo.toml

@@ -82,5 +82,5 @@ pageant = { version = "0.0.1-beta.3", path = "../pageant" }
 
 [dev-dependencies]
 env_logger = "0.11"
-tempdir = "0.3"
+tempfile = "3.14.0"
 tokio = { workspace = true, features = ["test-util", "macros", "process"] }

russh-keys/src/known_hosts.rs

@@ -194,7 +194,7 @@ mod test {
     #[test]
     fn test_check_known_hosts() {
         env_logger::try_init().unwrap_or(());
-        let dir = tempdir::TempDir::new("russh").unwrap();
+        let dir = tempfile::tempdir().unwrap();
         let path = dir.path().join("known_hosts");
         {
             let mut f = File::create(&path).unwrap();

russh-keys/src/lib.rs

@@ -834,7 +834,7 @@ Cog3JMeTrb3LiPHgN6gU2P30MRp6L1j1J/MtlOAr5rux
         env_logger::try_init().unwrap_or(());
         use std::process::Stdio;
 
-        let dir = tempdir::TempDir::new("russh")?;
+        let dir = tempfile::tempdir()?;
         let agent_path = dir.path().join("agent");
         let mut agent = tokio::process::Command::new("ssh-agent")
             .arg("-a")
@@ -901,7 +901,7 @@ Cog3JMeTrb3LiPHgN6gU2P30MRp6L1j1J/MtlOAr5rux
     #[cfg(unix)]
     fn test_agent() {
         env_logger::try_init().unwrap_or(());
-        let dir = tempdir::TempDir::new("russh").unwrap();
+        let dir = tempfile::tempdir().unwrap();
         let agent_path = dir.path().join("agent");
 
         let core = tokio::runtime::Runtime::new().unwrap();