Support stable rust for compiling the runtime (#13580)

* Support stable rust for compiling the runtime

This pull request brings support for compiling the runtime with stable Rust. This requires at least
rust 1.68.0 to work on stable. The code is written in a way that it is backwards compatible and
should automatically work when someone compiles with 1.68.0+ stable.

* We always support nightlies!

* 🤦

* Sort by version

* Review feedback

* Review feedback

* Fix version parsing

* Apply suggestions from code review

Co-authored-by: Koute <koute@users.noreply.github.com>

---------

Co-authored-by: Koute <koute@users.noreply.github.com>

substrate/primitives/io/src/lib.rs

@@ -19,7 +19,7 @@
 
 #![warn(missing_docs)]
 #![cfg_attr(not(feature = "std"), no_std)]
-#![cfg_attr(not(feature = "std"), feature(alloc_error_handler))]
+#![cfg_attr(enable_alloc_error_handler, feature(alloc_error_handler))]
 #![cfg_attr(
 	feature = "std",
 	doc = "Substrate runtime standard library as compiled when linked with Rust's standard library."
@@ -1643,7 +1643,7 @@ pub fn panic(info: &core::panic::PanicInfo) -> ! {
 }
 
 /// A default OOM handler for WASM environment.
-#[cfg(all(not(feature = "disable_oom"), not(feature = "std")))]
+#[cfg(all(not(feature = "disable_oom"), enable_alloc_error_handler))]
 #[alloc_error_handler]
 pub fn oom(_: core::alloc::Layout) -> ! {
 	#[cfg(feature = "improved_panic_error_reporting")]