Set rust-version = 1.61 in Cargo.toml

Cargo.toml

@@ -2,7 +2,7 @@
 name = "smoltcp"
 version = "0.8.1"
 edition = "2018"
-rust-version = "1.60"
+rust-version = "1.61"
 authors = ["whitequark <whitequark@whitequark.org>"]
 description = "A TCP/IP stack designed for bare-metal, real-time systems without a heap."
 documentation = "https://docs.rs/smoltcp/"