[llvm-branch-commits] [clang-tools-extra] [clangd] Add clangd 21 release notes (PR #152256)

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github-actions[bot] wrote:

@HighCommander4 (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix.  When you are done, please add the release:note label to this PR. 

https://github.com/llvm/llvm-project/pull/152256