[analyzer][docs] Admit that the cleanup attribute is not supported (#81834)

In fact, the cleanup attribute is only added to the CFG, but still
unhandled by CSA.
I propose dropping this false "support" statement from the docs.

Author: steakhal

clang/docs/ReleaseNotes.rst

@@ -1369,9 +1369,6 @@ New features
   of static analysis tools, such as the Clang Static Analyzer.
   `Documentation <https://clang.llvm.org/docs/AttributeReference.html#suppress>`__.
 
-- Added support for the ``cleanup`` attribute.
-  `Documentation <https://clang.llvm.org/docs/AttributeReference.html#cleanup>`__.
-
 - Support "Deducing this" (P0847R7). (Worked out of the box)
   (`af4751738db8 <https://github.com/llvm/llvm-project/commit/af4751738db89a142a8880c782d12d4201b222a8>`__)