docs/release-notes: note Azure SSH regression fix with new openssl

On versions of the openssl crate new enough to have parse2(), the legacy
parse() function is a wrapper that panics when parsing the Azure SSH key
certificate:

    thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', [...]/openssl-0.10.52/src/pkcs12.rs:42:31

We've switched to parse2() in the current codebase, but older versions
built with newer openssl will fail.  Note the fix.

See coreos/fedora-coreos-tracker#1492.

docs/release-notes.md

@@ -11,6 +11,7 @@ Major changes:
 
 Minor changes:
 
+- Fix SSH key fetching on Azure with `openssl` crate ≥ 0.10.46
 
 Packaging changes: