Fixes CI.

Author: mordante

libcxx/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp

@@ -12,7 +12,7 @@
 // UNSUPPORTED: c++03
 
 // TODO: Investigate these failures which break the CI.
-// UNSUPPORTED: clang-18, clang-19, clang-20
+// UNSUPPORTED: clang-18, clang-19, clang-20, clang-21
 
 // The Android libc++ tests are run on a non-Android host, connected to an
 // Android device over adb. gdb needs special support to make this work (e.g.