Add LLVM 21.1 to supported versions list

vcpkg installs LLVM 21.1.1 via the overlay, but the ARROW_LLVM_VERSIONS
list only went up to 19.1, causing CMake's FindLLVMAlt.cmake to reject
the installed LLVM as incompatible.

Use 21.1 (not just 21) to match CMake's version compatibility rules
which require MAJOR.MINOR format for proper matching.

Author: timhurskidremio

cpp/CMakeLists.txt

@@ -160,6 +160,7 @@ set(ARROW_DOC_DIR "share/doc/${PROJECT_NAME}")
 set(BUILD_SUPPORT_DIR "${CMAKE_SOURCE_DIR}/build-support")
 
 set(ARROW_LLVM_VERSIONS
+    "21.1"
     "19.1"
     "18.1"
     "17.0"