update release notes for 1.68.2

Author: pietroalbini

RELEASES.md

@@ -1,3 +1,14 @@
+Version 1.68.2 (2023-03-28)
+===========================
+
+- [Update the GitHub RSA host key bundled within Cargo](https://github.com/rust-lang/cargo/pull/11883).
+  The key was [rotated by GitHub](https://github.blog/2023-03-23-we-updated-our-rsa-ssh-host-key/)
+  on 2023-03-24 after the old one leaked.
+- [Mark the old GitHub RSA host key as revoked](https://github.com/rust-lang/cargo/pull/11889).
+  This will prevent Cargo from accepting the leaked key even when trusted by
+  the system.
+- [Add support for `@revoked` and a better error message for `@cert-authority` in Cargo's SSH host key verification](https://github.com/rust-lang/cargo/pull/11635)
+
 Version 1.68.1 (2023-03-23)
 ===========================