dependencies: Switch to pnpm.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

scripts/lib/clean_node_cache.py

@@ -1,4 +1,9 @@
 #!/usr/bin/env python3
+
+# TODO: After switching from yarn to pnpm, we no longer create
+# /srv/zulip-npm-cache or symlink node_modules, so this script can be
+# replaced with shutil.rmtree("/srv/zulip-npm-cache").
+
 import argparse
 import os
 import sys