Update Rust versions under test from 1.30.1 to 1.37.0 (#5538)

Fixes build errors from using newer crate versions with an old Rust version.

tests/docker/languages/Dockerfile.testing.rust.1_30_1 → tests/docker/languages/Dockerfile.testing.rust.1_37_0

-FROM rust:1.30.1-slim-stretch as base
+FROM rust:1.37.0-slim-stretch as base
 WORKDIR /code
 ADD . .
 RUN cp flatc_debian_stretch flatc

tests/docker/languages/Dockerfile.testing.rust.big_endian.1_30_1 → tests/docker/languages/Dockerfile.testing.rust.big_endian.1_37_0

-FROM rust:1.30.1-slim-stretch as base
+FROM rust:1.37.0-slim-stretch as base
 RUN apt -qq update -y && apt -qq install -y \
     gcc-mips-linux-gnu \
     libexpat1 \