ci: use beta, add secondary nightly ci.

examples/stm32u5/src/bin/blinky.rs

@@ -1,6 +1,5 @@
 #![no_std]
 #![no_main]
-#![feature(type_alias_impl_trait)]
 
 use defmt::*;
 use embassy_executor::Spawner;

examples/stm32u5/src/bin/boot.rs

@@ -1,6 +1,5 @@
 #![no_std]
 #![no_main]
-#![feature(type_alias_impl_trait)]
 
 use defmt::*;
 use {defmt_rtt as _, embassy_stm32 as _, panic_probe as _};

examples/stm32u5/src/bin/usb_serial.rs

@@ -1,6 +1,5 @@
 #![no_std]
 #![no_main]
-#![feature(type_alias_impl_trait)]
 
 use defmt::{panic, *};
 use defmt_rtt as _; // global logger