[OpenACC][Docs] Add a release note for Clang 21 (#145938)
This patch adds a release note that explains the current status of
OpenACC in Clang. Currently we cannot actually make an executable
because the OpenACC dialect of MLIR doesn't support any amount of
lowering to LLVM-IR, so the usefulness of OpenACC is entirely for
front-end related uses, such as tooling or semantic checking.
