Update rust edition to 2021

[stmcginnis]

Issue number:

Closes #1789

Description of changes:

The 2021 edition of Rust has been available in the Bottlerocket SDK since late 2021. Most crates in the project still set their edition to 2018 though.

This updates the declared edition to 2021. This brings various default language improvements.

Testing done:

Ran cargo make check to run linters and unit tests.
Build aws-k8s-1.24 variant and deployed new cluster. Verified cluster initializes successfully and deployed a pod to cluster to verify operation.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[bcressey]

In a clean checkout of the current develop branch, if I run this command in sources and tools:

cargo fix --edition --all-features --allow-dirty

It suggests this fix:

diff --git a/sources/api/apiserver/src/server/exec/child.rs b/sources/api/apiserver/src/server/exec/child.rs
index e50076e5..d87e4971 100644
--- a/sources/api/apiserver/src/server/exec/child.rs
+++ b/sources/api/apiserver/src/server/exec/child.rs
@@ -142,6 +142,7 @@ impl ChildHandles {
         // process too, or it'd stick around forever.  Perform the rest of initialization in a
         // closure so that we can kill the child easily on any error, returning the original value.
         (move || {
+            let _ = &child;
             // Now that the process is spawned, if we created a PTY, close its slave fd in the
             // parent process, or reads of the master side will block.
             if let Some(close_fd) = child_fds.close_fd {

Which is a conservative fix for the new disjoint captures in closures feature.

[bcressey]

cargo fix --edition-idioms didn't seem to find anything to change for the 2021 edition, FWIW.

[zmrow]

🎮