Upgrade to rust nightly 2022-05-10

Finally!

Update the rust-toolchain file to reference the new nightly which can
build for AVR again.  Also update CI to use the new version.  And
finally, drop the note in the README about the old version being
necessary.

.github/workflows/ci.yml

@@ -70,7 +70,7 @@ jobs:
         uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: nightly-2021-01-07
+          toolchain: nightly-2022-05-10
           override: true
           components: rust-src
       - name: Install avr-gcc, binutils, and libc

README.md

@@ -5,7 +5,7 @@ Hardware Abstraction Layer for AVR microcontrollers and common boards (for examp
 **This is a new version of `avr-hal`!  Older versions can be found in the `old` branch but will no longer get support.**
 
 ## Quickstart
-You need a nightly Rust compiler for compiling Rust code for AVR.  **Note**: Due to a regression, versions after `nightly-2021-01-07` are currently broken (see [#124](https://github.com/Rahix/avr-hal/issues/124)).  Please use that version of the compiler for now. The correct version will be installed automatically.
+You need a nightly Rust compiler for compiling Rust code for AVR.  The correct version will be installed automatically due to the `rust-toolchain.toml` file.
 
 On Ubuntu, you'll need to install dependencies:
 

rust-toolchain.toml

@@ -1,4 +1,4 @@
 [toolchain]
-channel = "nightly-2021-01-07"
+channel = "nightly-2022-05-10"
 components = [ "rust-src" ]
-profile = "minimal"
+profile = "minimal"