Specify MSRV in protobuf crate

[varungandhi-src]

The protobuf-codegen crate fails to compile
with Rust 1.71.0 due to code in this crate,
so mark the MSRV in this crate correctly,
based on bisecting across major Rust versions.

rust-version docs

Error message on compiling with older versions:

error[E0445]: crate-private trait `MapFieldAccessor` in public interface
   --> /Users/varun/.local/share/mise/installs/rust/1.71.0/registry/src/index.crates.io-6f17d22bba15001f/protobuf-3.7.1/src/reflect/acc/v2/map.rs:115:1
    |
17  |   pub(crate) trait MapFieldAccessor: Send + Sync + 'static {
    |   -------------------------------------------------------- `MapFieldAccessor` declared as crate-private
...
115 | / pub fn make_map_simpler_accessor_new<M, T>(
116 | |     name: &'static str,
117 | |     get_field: for<'a> fn(&'a M) -> &'a T,
118 | |     mut_field: for<'a> fn(&'a mut M) -> &'a mut T,
...   |
121 | |     M: MessageFull,
122 | |     MapFieldAccessorImpl<M, T>: MapFieldAccessor,
    | |_________________________________________________^ can't leak crate-private trait

error[E0446]: private type `MapFieldAccessorImpl<M, T>` in public interface
   --> /Users/varun/.local/share/mise/installs/rust/1.71.0/registry/src/index.crates.io-6f17d22bba15001f/protobuf-3.7.1/src/reflect/acc/v2/map.rs:115:1
    |
33  |   struct MapFieldAccessorImpl<M, T>
    |   --------------------------------- `MapFieldAccessorImpl<M, T>` declared as private
...
115 | / pub fn make_map_simpler_accessor_new<M, T>(
116 | |     name: &'static str,
117 | |     get_field: for<'a> fn(&'a M) -> &'a T,
118 | |     mut_field: for<'a> fn(&'a mut M) -> &'a mut T,
...   |
121 | |     M: MessageFull,
122 | |     MapFieldAccessorImpl<M, T>: MapFieldAccessor,
    | |_________________________________________________^ can't leak private type

Some errors have detailed explanations: E0445, E0446.
For more information about an error, try `rustc --explain E0445`.

Originally detected in sourcegraph/scip#284 (comment)