Specify MSRV

Fixes #1385

Cargo.toml

@@ -18,6 +18,7 @@ edition = "2021"
 keywords = ["ffi"]
 categories = ["development-tools::ffi", "api-bindings"]
 exclude = [ ".github", "book", "tools" ]
+rust-version = "1.77"
 
 # This is stricter about avoiding optional features in dependencies
 # such as 'syn'. We build this way such that CI spots any mistakes