Add TODO.
# Unreleased
+
+# 21.4.3
- [changed] Firestore now limits the number of concurrent document lookups it
will perform when resolving inconsistencies in the local cache (#1374).
+
+# 21.4.2
+- [changed] Removed Guava dependency from the SDK. This change is the first
+ step in eliminating crashes caused by apps that depend on the wrong flavor of
+ Guava (#1125).
+
+# 21.4.1
- [fixed] Fixed a performance regression introduced by the addition of
`Query.limitToLast(n: long)` in Firestore 23.3.1.
- [changed] Changed the in-memory representation of Firestore documents to
