Bump expected LLVM version to 21 (#23532)

Author: sbc100

tools/shared.py

@@ -59,7 +59,7 @@
 # This is currently set to v18 since this is the version of node available
 # in debian/stable (bookworm).
 MINIMUM_NODE_VERSION = (18, 0, 0)
-EXPECTED_LLVM_VERSION = 20
+EXPECTED_LLVM_VERSION = 21
 
 # These get set by setup_temp_dirs
 TEMP_DIR = None