add MSRV to the protobuf crate

PiperOrigin-RevId: 671124202
diff --git a/rust/cargo/Cargo.toml b/rust/cargo/Cargo.toml
index 7a389e7..2ed8756 100644
--- a/rust/cargo/Cargo.toml
+++ b/rust/cargo/Cargo.toml
@@ -11,6 +11,7 @@
 edition = "2021"
 links = "libupb"
 license = "BSD-3-Clause"
+rust-version = "1.74"
 
 [lib]
 path = "src/shared.rs"