CI: Bump lagom clang to version 21

[nico]

Catches lagom CI up with #25035, #25757, #26214.

---

Another attempt at unborking the mac CI.

[nico]

Currently fails with

-- The C compiler identification is unknown
-- The CXX compiler identification is unknown
CMake Error at CMakeLists.txt:3 (project):
  The CMAKE_C_COMPILER:

    /opt/homebrew/opt/llvm/bin/clang

  is not a full path to an existing compiler tool.

  Tell CMake where to find the compiler by setting either the environment
  variable "CC" or the CMake cache entry CMAKE_C_COMPILER to the full path to
  the compiler, or to the compiler name if it is in the PATH.


CMake Error at CMakeLists.txt:3 (project):
  The CMAKE_CXX_COMPILER:

    /opt/homebrew/opt/llvm/bin/clang++

  is not a full path to an existing compiler tool.

  Tell CMake where to find the compiler by setting either the environment
  variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
  to the compiler, or to the compiler name if it is in the PATH.

I tried both patching in #26074 as well as a simpler change (currently commit 2 on this PR), but those didn't help. Need to figure out what's going on and try again.

[nico]

Made less urgent by #26217, see the comment over there for a writeup. It'd still be good to figure out how to make this go, if only for Linux. But as mentioned on the other issue, maybe we want to switch the mac bot back over to brew if it works with it after this change.

[stale]

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