changelog: don't mark fiber_set_cancellable deprecation as breaking

Remove the "breaking change" mark, as it's highly unlikely that it will
break anything.

Follow-up #7166

NO_DOC=tarantool/doc#3132
NO_TEST=changelog

changelogs/unreleased/gh-7166-deprecate-fiber_set_cancellable.md

 ## feature/core
 
-* **[Breaking change]** `fiber_set_cancellable()` is deprecated and
-  now does nothing (gh-7166).
+* `fiber_set_cancellable()` C API function is deprecated and now does
+  nothing (gh-7166).