Update changelog for 2.18.4

Author: Dave Bartolomeo

CHANGELOG.md

@@ -17,6 +17,13 @@
      you know what to do).
 -->
 
+## Release 2.18.4 (2024-09-12)
+
+### New Features
+
+- C# support for `build-mode: none` is now out of beta, and generally available.
+- Go 1.23 is now supported.
+
 ## Release 2.18.3 (2024-08-28)
 
 - There are no user-facing changes in this release.