rust: Bump futures dependency

Update futures to v0.3.31.

Change-Id: I31cbfa50f0bf054caaa9efe9d1e0f6e9cdaebe6a
Reviewed-on: https://bluetooth-review.googlesource.com/c/bluetooth/+/2540
Reviewed-by: Marie Janssen <jamuraa@google.com>

rust/Cargo.toml

diff --git a/rust/Cargo.toml b/rust/Cargo.toml
index d1f23ca..4fbcdbd 100644
--- a/rust/Cargo.toml
+++ b/rust/Cargo.toml
@@ -22,7 +22,7 @@
 ## External dependencies
 assert_matches = "1.5.0"
 bitfield = "0.14.0"
-futures = "=0.3.30"
+futures = "=0.3.31"
 lazy_static = "1.4"
 log = { version = "0.4.22", features = [ "kv", "std" ] }
 parking_lot = "0.12.0"