rust-toolchain.toml: Upgrade to latest nighlty release

Upgrade rust toolchain to use nightly-2023-12-18 release.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>

Author: b49020

rust-toolchain.toml

@@ -18,6 +18,6 @@
 # Toolchain override for rustup
 
 [toolchain]
-channel = "nightly-2021-09-20"
+channel = "nightly-2023-12-18"
 components = ["rust-src"]
 targets = ["aarch64-unknown-linux-gnu", "arm-unknown-linux-gnueabihf"]