[clang] [clang][docs] mark v21 as released (PR #158629)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-aarch64-darwin` running on `doug-worker-5` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/27794

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: ExecutionEngine/OrcLazy/multiple-compile-threads-basic.ll' FAILED ********************
Exit Code: 2

Command Output (stdout):
--
# RUN: at line 1
/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli -jit-kind=orc-lazy -compile-threads=2 -thread-entry hello /Users/buildbot/buildbot-root2/aarch64-darwin/llvm-project/llvm/test/ExecutionEngine/OrcLazy/multiple-compile-threads-basic.ll | /Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root2/aarch64-darwin/llvm-project/llvm/test/ExecutionEngine/OrcLazy/multiple-compile-threads-basic.ll
# executed command: /Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli -jit-kind=orc-lazy -compile-threads=2 -thread-entry hello /Users/buildbot/buildbot-root2/aarch64-darwin/llvm-project/llvm/test/ExecutionEngine/OrcLazy/multiple-compile-threads-basic.ll
# .---command stderr------------
# | PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
# | Stack dump:
# | 0.	Program arguments: /Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli -jit-kind=orc-lazy -compile-threads=2 -thread-entry hello /Users/buildbot/buildbot-root2/aarch64-darwin/llvm-project/llvm/test/ExecutionEngine/OrcLazy/multiple-compile-threads-basic.ll
# |  #0 0x0000000101db10f4 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli+0x100f0d0f4)
# |  #1 0x0000000101daeea4 llvm::sys::RunSignalHandlers() (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli+0x100f0aea4)
# |  #2 0x0000000101db1bf4 SignalHandler(int, __siginfo*, void*) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli+0x100f0dbf4)
# |  #3 0x0000000182f63584 (/usr/lib/system/libsystem_platform.dylib+0x18047b584)
# |  #4 0x000001010190bb80
# |  #5 0x0000000101916fd0 llvm::orc::ExecutionSession::removeJITDylibs(std::__1::vector<llvm::IntrusiveRefCntPtr<llvm::orc::JITDylib>, std::__1::allocator<llvm::IntrusiveRefCntPtr<llvm::orc::JITDylib>>>) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli+0x100a72fd0)
# |  #6 0x0000000101916d80 llvm::orc::ExecutionSession::endSession() (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli+0x100a72d80)
# |  #7 0x00000001019a27f0 llvm::orc::LLJIT::~LLJIT() (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli+0x100afe7f0)
# |  #8 0x00000001019a717c llvm::orc::LLLazyJIT::~LLLazyJIT() (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli+0x100b0317c)
# |  #9 0x0000000100eae4c0 runOrcJIT(char const*) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli+0x10000a4c0)
# | #10 0x0000000100ea9bf0 main (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/lli+0x100005bf0)
# | #11 0x0000000182ba7154
# `-----------------------------
# error: command failed with exit status: -11
# executed command: /Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root2/aarch64-darwin/llvm-project/llvm/test/ExecutionEngine/OrcLazy/multiple-compile-threads-basic.ll
# .---command stderr------------
# | FileCheck error: '<stdin>' is empty.
# | FileCheck command line:  /Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root2/aarch64-darwin/llvm-project/llvm/test/ExecutionEngine/OrcLazy/multiple-compile-threads-basic.ll
# `-----------------------------
# error: command failed with exit status: 2

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/158629