Update locked frozendict version to 2.3.3 (#13352)

frozendict 2.3.3 includes fixes for memory leaks that get triggered during `/sync`.

docs/upgrade.md

@@ -114,6 +114,15 @@ vice versa.
 Once all workers are upgraded to v1.64 (or downgraded to v1.63), event
 replication will resume as normal.
 
+## frozendict release
+
+[frozendict 2.3.3](https://github.com/Marco-Sulla/python-frozendict/releases/tag/v2.3.3)
+has recently been released, which fixes a memory leak that occurs during `/sync`
+requests. We advise server administrators who installed Synapse via pip to upgrade
+frozendict with `pip install --upgrade frozendict`. The Docker image
+`matrixdotorg/synapse` and the Debian packages from `packages.matrix.org` already
+include the updated library.
+
 # Upgrading to v1.62.0
 
 ## New signatures for spam checker callbacks