message: [Rust] Specify Minimum Supported Rust Version
  Related to
diff: @@ -9,6 +9,7 @@ homepage = "https://google.github.io/flatbuffers/"
  repository = "https://github.com/google/flatbuffers"
  keywords = ["flatbuffers", "serialization", "zero-copy"]
  categories = ["encoding", "data-structures", "memory-management"]
  +rust = "1.51"
  [dependencies]
  smallvec = "1.6.1"
  