feat: Fix minimum supported cargo version to 1.77

This should ensure, that our Cargo.lock file stays at version 3
when using `cargo update` or dependabot.

wireguard-broker/Cargo.toml

@@ -8,6 +8,7 @@ description = "Rosenpass internal broker that runs as root and supplies exchange
 homepage = "https://rosenpass.eu/"
 repository = "https://github.com/rosenpass/rosenpass"
 readme = "readme.md"
+rust-version = "1.77"
 
 [dependencies]
 thiserror = { workspace = true }