Add changelog entry for dead-variant removal

Author: JonoPrest

CHANGELOG.md

@@ -54,6 +54,7 @@
 - Cache OPAM env, rewatch build, and instrumented dune state in the coverage workflow. https://github.com/rescript-lang/rescript/pull/8434
 - Add a multi-file fixture harness (`super_errors_multi`) for cross-module errors and warnings. https://github.com/rescript-lang/rescript/pull/8433
 - Catalog every named compiler error variant in `tests/ERROR_VARIANTS.md` and add fixtures for the remaining reachable ones. https://github.com/rescript-lang/rescript/pull/8446
+- Remove dead and unreachable compiler error and warning variants; add fixtures for the ones found to be reachable. https://github.com/rescript-lang/rescript/pull/8459
 
 
 # 13.0.0-alpha.4