In trying to upgrade my mempool instance, I discovered I couldn't
build the latest mempool Rust code with my available Rust
toolchain.
It appears in #4612 the Rust MSRV was bumped without justification,
which is reverted here. Note that `Cargo.lock` updates here should
ensure the versions of dependent crates use the versions supported
by our MSRV.
Its possible that the dependency downgrades here break something,
but things appear to be running fine for me locally, so figured I'd
suggest this upstream.
