Set minimal supported Rust version to 1.75

Current code has `Option::is_none_or()` which released 2024 Oct 17,
considering old compiler environment like Ubuntu 24.04 LTS, RHEL 9,
replaced it to:

```rust
self.unsolicited_messages_tx.as_ref().is_none()
    || self.unsolicited_messages_tx.as_ref().map(|x| x.is_closed())
```

Set minimum supported Rust version to 1.75 to `Cargo.toml` and enabled
CI test for it.

Signed-off-by: Gris Ge <[email protected]>

Author: cathay4t

Cargo.toml

@@ -3,6 +3,7 @@ authors = ["Corentin Henry <[email protected]>"]
 name = "netlink-proto"
 version = "0.11.4"
 edition = "2018"
+rust-version = "1.75"
 
 homepage = "https://github.com/rust-netlink/netlink-proto"
 keywords = ["netlink", "linux", "async"]

src/connection.rs

@@ -236,11 +236,13 @@ where
             }
         }
 
+        // Rust 1.82 has Option::is_none_or() which can simplify
+        // below checks but that version is released on 2024 Oct. 17 which
+        // is not available on old OS like Ubuntu 24.04 LTS, RHEL 9 yet.
         if ready
-            || self
-                .unsolicited_messages_tx
-                .as_ref()
-                .is_none_or(|x| x.is_closed())
+            || self.unsolicited_messages_tx.as_ref().is_none()
+            || self.unsolicited_messages_tx.as_ref().map(|x| x.is_closed())
+                == Some(true)
         {
             // The channel is closed so we can drop the sender.
             let _ = self.unsolicited_messages_tx.take();