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/utils/wasm-builder/README.md

@@ -77,8 +77,13 @@ Wasm builder requires the following prerequisites for building the Wasm binary:
 
 - rust nightly + `wasm32-unknown-unknown` toolchain
 
-If a specific rust nightly is installed with `rustup`, it is important that the wasm target is installed
-as well. For example if installing the rust nightly from 20.02.2020 using `rustup install nightly-2020-02-20`,
-the wasm target needs to be installed as well `rustup target add wasm32-unknown-unknown --toolchain nightly-2020-02-20`.
+or
+
+- rust stable and version at least 1.68.0 + `wasm32-unknown-unknown` toolchain
+
+If a specific rust is installed with `rustup`, it is important that the wasm target is
+installed as well. For example if installing the rust from 20.02.2020 using `rustup
+install nightly-2020-02-20`, the wasm target needs to be installed as well `rustup target add
+wasm32-unknown-unknown --toolchain nightly-2020-02-20`.
 
 License: Apache-2.0